Self adjoint test running Forward then Backward with different rhs terms

[rmadec-cs]

acoustic and elastic

[Bubusch]

should it be moved to tests/validation folder since it looks like a validation test more than an example on how to use the library ? and run with commands like pytest tests/validation --order 2 --mesh cartesian --tolerance 1e-2. tThis will also allow to easily assert on some checks.

this will imply replacing argparse by pytest parser to get something like

def pytest_addoption(parser):
    parser.addoption("--order", type=int, default=1)
    parser.addoption("--mesh", type=str, default="cartesian")
    parser.addoption("--elastic", action="store_true", default=False)
    parser.addoption("--tolerance", type=float, default=5e-2)