Sf2sh second try at correcting suggestions

- tests for the real_sph_harm_mrtrix and fibernav fonctions
- brought tests that were in test_odf.py in test_shm.py to test sf_to_sh and sh_to_sf functions
- PEP8 compliance for test_shm
- integrated stefanv's modifications

@MrBago Your idea about changing the parameters to the real_shm_harm using sh_order instead of the m,n is a good idea and you can go ahead and do it. But this means that all the tests will have to be adjusted accordingly.