418 read/write of flags in GBTFITSLoad

[mpound]

This PR adds

• reading of GBTIDL flag files and creation of Flags object (sdf.flags) and data mask therefrom (sdf._sdf[i]._flagmask[k])
• the GBTFITSLoad constructor has a skipflags=T/F to control reading of GBTIDL flags files [default: False]
• Writing of flags to SDFITS via GBTFITSLoad.write(flags=True). flags=True is the default. A new column "FLAGS" with type unsigned int (1 or 0) and shape identical to the "DATA" column is added to the binary table HDU.
• new tests of FLAGS read/write and updates of old tests where necessary. Tests include both multifile=True and False and flags=True and False.

This PR does NOT add:

• writing of flags from Scans for SDFITS output
• writing of flags directly from SDFITSLoad. Although there is a flags argument in SDFITSLoad.write, it is currently ignored. In our case the FLAGS column is added before write() is called, so we don't want to double-add it. We can think about how important it is to implement also in SDFITSLoad.

[astrofle]

The reading of GBTIDL flags will fail if the flag was created after the fact through GBTIDL as it adds a space to the line, so the first character becomes ' *' rather than '*' (fixed adding v.strip()).

It is also possible to create flags that include ranges so the flag file could look like:

[header]
created = Fri Nov 22 19:04:55 2024                                                                                                                                                                               
version = 1.0                                                                                                                                                                                                    
created_by = gbtidl                                                                                                                                                                                              
[flags]
#RECNUM,SCAN,INTNUM,PLNUM,IFNUM,FDNUM,BCHAN,ECHAN,IDSTRING
 *|20|42:51|*|*|*|2299|*|unspecified

This also implies that the flag only contains a start channel (BCHAN) but no end channel, which is currently not supported by the reader.

[astrofle]

Not strictly related to this PR, but while trying to write a how-to guide I ran into these issues:
#429
#428
#427

[astrofle]

Thanks a lot for all the work!
The only outstanding issue flag related is #432 (that is documented).
Let me know how you'd like to proceed.

[mpound]

All three issues are now fixed. I identified one more during this testing. I will make an issue but it should not hold up this merge.