  Here is the Python code based on the given instructions:
```
import porepy as pp
import numpy as np

# Define two functions for adding data and plotting over a line
def add_data(grid_bucket, domain):
    # Define permeability, apertures, and boundary conditions
    grid_bucket.permeability = np.ones(grid_bucket.num_cells)
    grid_bucket.apertures = np.zeros(grid_bucket.num_cells)
    grid_bucket.boundary_conditions = np.zeros(grid_bucket.num_cells)

def plot_over_line(grid_bucket, line):
    # Plot values over a line in a grid bucket
    pass

# Set tolerance value and mesh size parameters
tolerance = 1e-6
mesh_size = 0.1

# Import a grid bucket from a CSV file, compute its geometry, coarsen it, and assign node ordering
grid_bucket = pp.GridBucket.from_csv("grid_bucket.csv")
grid_bucket.compute_geometry()
grid_bucket.coarsen(mesh_size)
grid_bucket.assign_node_ordering()

# Use the add_data function to assign parameters to the grid bucket
add_data(grid_bucket, domain)

# Define a solver using the DualVEMMixDim class for flow
solver = pp.DualVEMMixDim(grid_bucket, domain)

# Compute the matrix and right-hand side of the system, and solve it
solver.compute_matrix()
solver.compute_rhs()
solver.solve()

# Split the solution, extract the discharge and pressure, and project the discharge
solution = solver.split_solution()
discharge = solution.discharge
pressure = solution.pressure
projected_discharge = solver.project_discharge(discharge)

# Export the grid bucket to a VTK file, including the pressure and discharge
pp.GmshGridBucketWriter.write_grid_bucket(grid_bucket, "grid_bucket.vtk", pressure, discharge)

# Define a bounding box and a number of points, and create two sets of points along the x and y axes
bounding_box = pp.BoundingBox(0, 1, 0, 1)
num_points = 100
x_points = np.linspace(bounding_box.x_min, bounding_box.x_max, num_points)
y_points = np.linspace(bounding_box.y_min, bounding_box.y_max, num_points)

# Use the plot_over_line function to plot the pressure along these lines and save the results to CSV files
for x in x_points:
    for y in y_points:
        pressure_line = pp.Line(x, y, bounding_box)
        pressure_line.plot_over_line(grid_bucket, pressure, "pressure_line_{}_{}.csv".format(x, y))

# Print the diameter of the grid bucket and the number of cells in 2D and 1D
print("Diameter of grid bucket:", grid_bucket.diameter)
print("Number of cells in 2D:", grid_bucket.num_cells_2d)
print("Number of cells in 1D:", grid_bucket.num_cells_1d)
```