Consider merging other FITS column and unit code into desiutil.annotate

[weaverba137]

While working on desihub/desispec#2176, we created or encountered other code to manipulate FITS BinTableHDU columns descriptions and units in desispec.io.util.write_bintable and desispec.io.fibermap.annotate_fibermap. These functions have different high-level functionality, but some low-level code is almost certainly in common with desiutil.annotate.annotate_fits.

• desiutil.annotate.annotate_fits is for updating an existing BinTableHDU in an old file, while taking care to not alter the data in other HDUs.
• desispec.io.util.write_bintable is for writing or appending a BinTableHDU, but the input is not guaranteed itself to be a BinTableHDU.
• desispec.io.fibermap.annotate_fibermap uses a pre-defined list of columns to manipulate the output of desispec.io.fibermap.assemble_fibermap and other FIBERMAP HDUs in the pipeline.

As part of this we may want to consider moving write_bintable entirely into desiutil since there is nothing really specific to desispec about it.

[moustakas]

@weaverba137 were all these tasks done in #201? If so, can we close this ticket?

[weaverba137]

No, and in fact, this issue was created after #201 was closed.