Add support for standard healpix fits files

[bhazelton]

The standard healpix fits file format uses a binary table which by definition in FITS can only have 2 axes. What is currently implemented in beamfits.py uses image-type extensions which can have as many axes as needed.

Healpix files currently can contain all 4 polarizations only if they are ‘full sky’ images. If they are ‘cut sky’ images — meaning you only have some of the pixels, you need a column for the pixel index. And since healpix fits binary tables can only have up to 4 columns, that means you have to put each polarization in it’s own file.

So to convert to standard healpix FITS, each file or HDU can only be for a single frequency, pol and vector index. This can be accomplished with an export function in beamfits that uses select to break out parts of the beam and then put them in separate HDUs or files. Then to read them in we can use the add method.