Tools for calibrating photutils outputs

Prompted by the question in #433 (although this has been discussed in various other places), we should develop some functions to do standard sorts of calibrations given the outputs of `photutil`'s aperture and PSF photometry elements.

The way I imagine this working is basically a set of functions that take tables as they are output from the aperture photometry routines or PSF photometry classes, applying various calibrations. The minimal example is something like this:
```python
def calibrate_magnitudes(tab, zeropoint):
tab['cal_mags'] = -2.5*np.log10(tab['flux_fit'])*u.mag + zeropoint
```

But a much wider set of tools are called for, including:
* Take an output table with `SkyCoord`s, and use that to match some standard star catalogs (which might or might not be provided in the package), and use that to estimate photometric zero points
* Use bright stars to do aperture corrections from flux measurements
* Combining *multiple* catalogs from different exposures, matching up objects that are likely the same, and computing the combined magnitudes/uncertainties

These are all quite distinct tasks, but I think we want all of them, at least at some point.

Start by reading issue #433 and reviewing the aperture-photometry output tables and PSF photometry classes referenced here. The scope needs to be narrowed among magnitude calibration, standard-catalog matching, aperture corrections, and combining catalogs before implementation; done would mean an agreed set of tools with corresponding behavior and tests.

Written by the indexing model from the issue text.