FIX: make_inverse_operator checks
ENH: make_inverse_operator

automatically set loose = None for fixed = True
FIX: make_inverse_operator

don't assume that loose=None entails fixed=True
ENH: make_inverse_operator

Allow surf_ori=False forward operator with loose=1
FIX: read_inverse_operator()

Conditions for setting the depth_prior to None in the returned
dictionary
TEST: add test_make_inverse_operator_free()
OPT: compute_orient_prior

skip multiplication with 1
DOC
RF: compute_orient_prior; pep8
gitignore: add "coverage"
TEST: add class with setup method to avoid reading fwd and inv op from file multiple times
FIX TEST: inv op test class