Continuous modelling: SHORE

Hello all, 
this is a PR that Sylvain Merlet started, and I extended with implementing the analytical pdf, analytical return to origin probability and analytical mean square displacement. 
This analytical model is able to fit any type of HARDI acquisition (single-shell, multi-shell or DSI-like) and calculate traditional features like the PDF or the ODF. I am not sure about its performance with single-shell data but it works well with multi-shell and DSI data.
Thanks to the continuous nature of the model metrics like MSD and RTOP (which calculate integral over all the q or the r-space) look better than the standard DSI ones.
I also implemented in the simulations module the multi-tensor pdf, rtop and msd for comparisons.

The continuous analytic model used here is the SHORE which can be thought as an extension of the SH basis from 2D to 3D space.

Thanks, waiting for your comments!