LSMR and BiCG currently failing tests for complex matrices

[campospinto]

The LSMR and BiCG solvers are currently failing the tests for complex matrices: the tests will be skipped in psydac/linalg/tests/test_solvers.py and a NotImplementedError raised in the inverse factory method, but this should be investigated.

[campospinto]

As suggested in the discussion of PR #512 we could have a list of implemented solvers as module variable (in linalg.solvers) or class attribute (in InverseLinearOperator) that is read in the tests, so that no solver is left untested.