Pixel API

There are currently quite a few places both in here and in the image library where we do clunky things to get at, operate on, and assign sub pixels of a given Pixel type.

We need at least:

• Type level operations on Pixels, e.g. get the equivalent type where all channels are of a given floating point type,
• A wider set of arithmetic operators available on Pixels (so that we can just add p and q, rather than get all their channel values, add those, and set as channels of a pixel r),
  • A handy set of bounds to express when certain operations are possible, e.g. expressing when filtering by a float-valued kernel is appropriate.

Try to replace all current iterations over pixel channels with something nicer.

The issue names Pixel operations in imageproc and the image library but no files or tests. Start by surveying current iterations over pixel channels and the existing Pixel types. Done means providing type-level channel operations, broader Pixel arithmetic, useful operation bounds, and replacing the current channel iterations.