ca.py: implementation of RST-AM;
pca.py: implementation of RST-AM with preconditioning.

Note: We originally called the RST-AM method as Conjugate Anderson Mixing (CA), thus the source code name is ca.py.
RST-AM with preconditioning was originally called as preconditioned Conjugate Anderson Mixing (PCA), thus the source code name is pca.py.

