[WIP] Improvements to reading / writing FITS images

[jzuhone]

This PR introduces improvements to FITS file writing and reading. Specifically:

1. Specifies a new standard for writing metadata to FITS files using the FITSImageData class and its subclasses. This standard uses the header to write unit information and other metadata to the FITS file so that when it is read back into yt as a dataset it is more self-describing.
2. Defines a new class, YTFITSDataset, which is a subclass of FITSDataset which looks specifically for this header metadata.
3. Simplifies the logic in FITSHierarchy for determining units.
4. Makes the "beam" unit part of SkyDataFITSDataset.
5. Adds a convolve method to FITSImageData instances to convolve them with any kernel created using AstroPy or a simple Gaussian kernel.
6. Specifies that the default image resolution for FITSImageData subclasses should be 512. The previous behavior was inconsistent across the various subclasses.

The new yt-based standard for writing FITS datasets with self-describing information and reading them back in as datasets is set out in a YTEP:

yt-project/ytep#5

Things left to do:

• add built-in method to YTCoveringGrid to write these grids to FITS files
• change docs
• fix / add more tests
• ensure consistency with analysis modules

[jzuhone]

@yt-fido test this please

[jzuhone]

Closing this because I've rebased on yt-4.0.