Fixel Analysis: fitting SH-FOD with parametric distributions

[rutgerfick]

Extension of FittedMCSH model to fit the spherical FOD using one or more parametric distributions (watson/bingam), essentially making a bridge to standard MC models. This way you can get an estimate of the dispersion in multiple crossings from e.g. MT-CSD without having to do inefficient NODDIx stuff.