Better PSF Photometry

[StuartLittlefair]

This PR addresses #106 by adding non-circular PSFs to the pipeline using photutils.

PSF Photometry is available as an option in reduce. It is turned on and off from settings in the reduce.red file. PSF Photometry is a three-step process:

• in the first step the standard aperture fitting and repositioning is carried out, as for normal reduction;
• in the second step the reference stars are fit to fix the PSF shape;
• lastly, the PSF model is fit to the image with a source at each aperture position but the shape held fixed.

Because we can't make a contiguous image across window boundaries, PSF Photometry currently only works if all apertures are within a single window.

The PR also adds a new script psfaper which will help create aperture files for crowded regions. In psfaper the user is asked to zoom in on the region where they want to carry out PSF Photometry. The user then adds a small number of reference apertures and hits 'q'.

The reference stars are then fit to constrain the shape of the PSF. With the PSF shape held fixed, the script then carries out multiple iterations of FIND-FIT-SUBTRACT to detect all sources in the region of interest, these are then added to the aperture file.

There is still work to do here, most significantly it needs documentation both inside the code and in the main docs to explain PSF photometry. However I think it is ready for a code review and some stress testing.

[StuartLittlefair]

Edit: I've now added docs and fixed some bugs so it is ready for full code review