Add csd plus

[tomdelahaije]

This request implements the CSD+ constraints proposed in https://doi.org/10.1016/j.neuroimage.2019.116405, discussed also in #91. The constraints can be used by setting solver='csd_plus' when fitting the model. I've included a simple example based on the existing MC-CSD example.

It is recommended to install the MOSEK solver for speed, cf. https://docs.mosek.com/latest/install/installation.html for installation.

Some points:

• The maximum value for sh_order when using the constraints is currently 10, but this is not checked anywhere.
• The precomputed constraint matrices are stored as CSV files in dmipy/data/sos_constraints, but there is probably a more efficient way of storing/reading them.