Implement suppressive surround as a DoG

[MSchnei]

• This can be implemented by looping around pyprf_feature and establishing the best fit per voxel in several loops (see pyprf_motion; flag --cmpr)
• In the first loop, the standard model without any suppressive surround is fit; this will serve as a baseline reference for future fits
• In the following loop, different ratios of the surround size with regard to the center size are tested.
• Zuiderbaan et al. can serve as a reference article for which ratios to choose.

[MSchnei]

This feature has been implemented.