Fixed the default dipy SH basis not to be the fibernav one

Maybe I have missed this in previous discussions with @MrBago and @stefanv about SH bases (dipy, mrtrix, fibernav) a while back. However, currently, the sph_harm_lookup table was pointing to the fibernav basis by default. Was this intentional? If so, that is fine and we can make it the default one.

If not intentional, I have added a real_sph_harm_dipy with the same signature as the mrtrix and fibernav one. This way, it can be used in the lookup table. 

However, it duplicates code of the real_sph_harm, which might have still been there for backwards compatibility. 

Added a test as well.