enhancement: linear interpolation in reslicing. bugfix: in reslicing (ZSurface_reslice2) using extendZeros can hurt the mincostsurface detection, extendBorder is used now

Add 3 ops to ease scripting:  one to create a z map, one to create two z map with no crossing constraints and one to reslice a volume along a z map.

Update image type, and change ZMapReslice(...) to fill empty region with zeeros (rather than extending border)