Make fitmap search use random placements within the target volume envelope

[Tom Goddard]

Roden Luo the developer of DiffFit points out that choosing the center point for randomly placing the structure to fit using fitmap search uniformly within the bounding box of the displayed target volume envelope is pretty inefficient. This adds a lot of volume outside the envelope of the target volume at the current threshold. It would be better to uniformly sample within the envelope of the target volume so fewer search positions would be needed.