Yes, I took the mask out b/c as is the prf classes only handle a single voxel at a time.  The mask would be sort of useless.  

BUT, this does speak to the larger issue of how to use the prf module with multiple voxels of data that need to be fit.  I assume each voxel needs to have its own instantiation of GaussianFit(), which would render a mask useless within that class, no?