Remove default xInterp, kInterp from C++ layer SBInterpolatedImage
(#476)
Change default pad_factor from 0 to 4 and require pad_factor > 0.  i.e. don't reinterpret pad_factor <= 0 as pad_factor = 4.
(#476)
Workaround for clang warning when MEM_TEST=true
(#476)