Read/Write of flags in SDFITS, Scan, and Spectrum

[mpound]

Feature description
Requirements are the abilities to

1. Read GBTIDL flags
2. Write applied flags/mask back out from the classes which we support data-writing.
3. Read flags/masks that have been written to our classes.

Solution

1. Figure out how to parse GBTIDL flag files into a Flag object, which is then attached to the GBTFITSLoad object
2. For SDFITS format we would write a new binary table column "FLAGS" that has same dimensions as the "DATA" column and consists of a bitmask. Possibly using https://docs.astropy.org/en/stable/nddata/bitmask.html. For Spectra, there is already a column with the mask value.
3. Creating a Flag object from SDFITS bitmask may be tricky. Brute force is to make one Flag._flag_selection_rule per row. Channel flags themselves are stored per binary table in SDFITSLoad._flagmask.

Additional context
We will not write out GBTIDL flag format. Also VEGAS flags are covered under issue #30