Fix FITS file resizing issues

[embray]

This is a sort of generic, broad issue as a reminder that there are still bugs that can occur when resizing FITS files.

Many of these bugs have to do with the fact that HDU objects contain bookkeeping such as the offset into the FITS file where that HDU's data starts. So if a file is opened in 'updated' mode, and some changes are flushed that result in the file having to be resized, not all that book keeping is kept properly up to date.

This relates to some of my desire to change the underlying I/O model for FITS files, where HDUs would be made more abstract, and I/O details about individual physical files would be handled at a different layer.