Python bindings for ImageBufAlgo

The vast majority of ImageBufAlgo functions are hereby exposed to Python. (Including docs and unit tests!)

A small number of functions have temporarily been omitted.  These are all rarely used, and had call signatures where I was unsure of the correct Pythonic way to organize them.  We'll come back to this later, but in the mean time, we have now working versions of almost everything else in ImageBufAlgo that you can use from Python.