NPI-3689 Restructure IAU2000 download functionality

[treefern]

Separate IAU2000 file variant selection logic from download functionality and improve selection logic to allow passing a date range rather than a single date.

[ronaldmaj]

I've discovered a couple errors coming up that I think shouldn't be, based on my inputs?
Something to look at next week perhaps

[ronaldmaj]

I seem to hit this error whenever I put through a start date older than 90 days:

In [23]: download_iau2000_file(download_dir=dwn_dir, start_epoch=st2)
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
Cell In[23], line 1
----> 1 download_iau2000_file(download_dir=dwn_dir, start_epoch=st2)

File /data/gnssanalysis/gnssanalysis/gn_download.py:1094, in download_iau2000_file(download_dir, start_epoch, if_file_present)
   1092 variants = get_iau2000_file_variants_for_dates(start_epoch=start_epoch)
   1093 if len(variants) != 1:
-> 1094     raise NotImplementedError(
   1095         "Legacy wrapper for IAU2000 file download failed. Exactly one file variant should be returned based on "
   1096         f"a single date ({start_epoch})."
   1097     )
   1098 variant = variants.pop()
   1099 download_iau2000_variant(download_dir=download_dir, iau2000_file_variant=variant, if_file_present=if_file_present)

NotImplementedError: Legacy wrapper for IAU2000 file download failed. Exactly one file variant should be returned based on a single date (2024-10-23 06:00:00).

When I run the get_iau2000_file_variants_for_dates() function with just the start_epoch above, I get back two variants (rather than one)

[ronaldmaj]

When I run this with a single date (a start_epoch) a little over 90 days ago, I get two variants back, rather than one