  Here is the Python code based on the given instructions:
```
import numpy as np
import scipy.sparse as sp
import os
import sys
from porepy import DarcyAndTransport, test_ismember_rows_double_occurence_a_and_b, _a_in_b, test_round_up_and_down, _point_and_edge_lists, TestGettersAndSetters, _uniquify_points_and_edges, test_many_points_and_segments, test_single_point_and_segment, test_one_cell_a_time_node_keyword, Parameters, snap_to_grid, check_parameters, map_subgrid_to_grid, SnapToGridTest, test_ismember_rows_double_occurence_a_no_b, apply_function_to_edges, apply_function_to_nodes, to_vtk, to_gmsh

def add_data_darcy(gb, tol):
    # Add parameters related to Darcy's law
    gb.add_parameter('permeability', 1.0)
    gb.add_parameter('source', 0.0)
    gb.add_parameter('aperture', 0.0)
    gb.add_parameter('boundary_conditions', [])

def add_data_advection(gb, tol):
    # Add parameters related to advection
    gb.add_parameter('source', 0.0)
    gb.add_parameter('porosity', 0.0)
    gb.add_parameter('discharge', 0.0)
    gb.add_parameter('boundary_conditions', [])

def solve_darcy(gb, tol):
    # Create a grid using the soultz_grid module
    grid = soultz_grid.create_grid(gb.parameters['permeability'], gb.parameters['aperture'], gb.parameters['boundary_conditions'])

    # Compute the geometry of the grid
    grid.compute_geometry()

    # Coarsen the grid if a certain condition is met
    if grid.coarsen_grid():
        grid.compute_geometry()

    # Assign node ordering to the grid
    grid.assign_node_ordering()

    # Solve a Darcy problem using the DualVEMMixDim solver from the porepy library
    solver = DarcyAndTransport(grid, gb.parameters['permeability'], gb.parameters['aperture'], gb.parameters['boundary_conditions'])
    solver.solve()

    # Extract discharge and pressure from the solution
    discharge = solver.get_discharge()
    pressure = solver.get_pressure()

    # Project discharge onto the grid
    discharge_projected = solver.project_discharge(discharge)

    # Compute the total flow rate
    total_flow_rate = np.sum(discharge_projected)

    return total_flow_rate

def solve_advection(gb, tol):
    # Create a grid using the soultz_grid module
    grid = soultz_grid.create_grid(gb.parameters['porosity'], gb.parameters['discharge'], gb.parameters['boundary_conditions'])

    # Compute the geometry of the grid
    grid.compute_geometry()

    # Coarsen the grid if a certain condition is met
    if grid.coarsen_grid():
        grid.compute_geometry()

    # Assign node ordering to the grid
    grid.assign_node_ordering()

    # Define solvers for advection and mass matrix
    advection_solver = test_ismember_rows_double_occurence_a_and_b(grid, gb.parameters['porosity'], gb.parameters['discharge'], gb.parameters['boundary_conditions'])
    mass_matrix_solver = test_round_up_and_down(grid, gb.parameters['porosity'], gb.parameters['discharge'], gb.parameters['boundary_conditions'])

    # Compute matrices and right-hand side vectors
    A = advection_solver.compute_matrix()
    b = advection_solver.compute_right_hand_side_vector()
    M = mass_matrix_solver.compute_matrix()
    f = mass_matrix_solver.compute_right_hand_side_vector()

    # Perform an LU factorization
    lu = sp.linalg.lu_factor(A)

    # Initialize a solution vector
    x = np.zeros(A.shape[0])

    # Perform a time-stepping loop to update the solution and export it at certain time steps
    for i in range(100):
        # Update the solution
        x = sp.linalg.lu_solve(lu, b)

        # Export the solution at certain time steps
        if i % 10 == 0:
            to_vtk(grid, x, 'solution_%d.vtk' % i)

    return x

if __name__ == '__main__':
    # Set up parameters for creating a grid
    parameters = Parameters()
    parameters.add_parameter('permeability', 1.0)
    parameters.add_parameter('aperture', 0.0)
    parameters.add_parameter('boundary_conditions', [])

    # Set up parameters for a transport problem
    parameters.add_parameter('porosity', 0.0)
    parameters.add_parameter('discharge', 0.0)
    parameters.add_parameter('boundary_conditions', [])

    # Create a grid using the soultz_grid module
    grid = soultz_grid.create_grid(parameters)

    # Compute the geometry of the grid
    grid.compute_geometry()

    # Coarsen the grid if a certain condition is met
    if grid.coarsen_grid():
        grid.compute_geometry()

    # Assign node ordering to the grid
    grid.assign_node_ordering()

    # Add data to the grid bucket
    gb = GridBucket(grid)
    add_data_darcy(gb, 1e-6)
    add_data_advection(gb, 1e-6)

    # Solve a Darcy problem
    total_flow_rate = solve_darcy(gb, 1e-6)

    # Solve an advection problem
    solution = solve_advection(gb, 1e-6)

    # Export the solution in PVD format
    to_pvd(grid, solution, 'solution.pvd')

    # Save the production data to a text file
    with open('production_data.txt', 'w') as f:
        f.write('Total flow rate: %f\n' % total_flow_rate)
        f.write('Solution: %s\n' % solution)
```