Introduce PixelCoordinates class

[cdeil]

As discussed in today's photutils hangout (see minutes), we propose to introduce a PixCoord class and use it throughout photutils.

This is a first step towards a simple and consistent API. Currently e.g. the aperture_photometry function takes two arguments xc and yc and the psf_photometry function (to be merged today) takes a single argument positions. Soon we also want to support SkyCoord coordinates, at least in some convenience functions ... this will be discussed in a separate issue later.

Once Generalised WCS becomes available in Astropy it will probably contain a class similar to PixCoord and we'll use it, but for now we'll just make our own in photutils.

cc @astrofrog @eteq @bsipocz

[cdeil]

Has been discussed and rejected ... we'll keep using tuples for pixel coordinates.