Added sampling API docs

I've added API docs for the cython `downsample` and `upsample` functions, but they are not being picked up in the documentation by `automodapi`.  Any ideas @cdeil, @astrofrog, @eteq?

This is the API docs part of https://github.com/astropy/photutils/issues/73
Use the term `summing` instead of `combining` to be more specific?