Add zero_tol variable to mps classes. Pass on to functions that use it.
Some cleanups and removal of a debug line in tdvp_common.
ppinv solver: Improve sanity checks, make sparse solver configurable, add unit tests.
Use gmres solver by default for pseudo-inverses in excitations code.
Also be consistent about conjugates in 2-site excitations code
and clean it up a little.
Improve doc string for eps_l.