NF: added MRtrix and FiberNavigator SH bases
NF: added sh_estimate executable to estimate SH coeffcients with sh_to_sf and sf_to_sh functions in shm.py
added a comment
TEST: wrote tests for sh_to_sf and sf_to_sh functions
DOC: added documentation for the sh_to_sf and sf_to_sh, and corrected the help message in bin/sh_estimate.py
addressed all pull requests suggestions
Merge pull request #139 from mdesco/sf2sh

Spherical function to spherical harmonics and back