fix incorrect asteroid-comet classification bug

[hhsieh00]

Fixing a bug where parse_comet() incorrectly interprets input like "2015XN77" as a well-formed comet designation (where number=2015, type=X, and name=N77)

[github-actions]

Thank you for your contribution to sbpy, an Astropy affiliated package! 🌌 This checklist is meant to remind the package maintainers who will review this pull request of some common things to look for.

• Do the proposed changes actually accomplish desired goals?
• Do the proposed changes follow the sbpy coding guidelines?
• Are tests added/updated as required? If so, do they follow the Astropy testing guidelines?
• Are docs added/updated as required? If so, do they follow the Astropy documentation guidelines?
• Is rebase and/or squash necessary? If so, please provide the author with appropriate instructions. Also see instructions for rebase and squash.
• Did the CI pass? If no, are the failures related?
• Is a change log needed? If yes, did the change log check pass? If no, add the "no-changelog-entry-needed" label.
• Is a milestone added?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.63%. Comparing base (9360983) to head (dcc38dc).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #422      +/-   ##
==========================================
+ Coverage   84.61%   84.63%   +0.01%     
==========================================
  Files          92       92              
  Lines        8420     8426       +6     
==========================================
+ Hits         7125     7131       +6     
  Misses       1295     1295              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[hhsieh00]

So the problem in issue #421 seems to actually be due to the second part of the asteroid designation also being a potential comet type, so parse_comet will interpret "2015XN77", "2015PN77", and "2015CN77" all as comets (broken down as number=2015, type=X/P/C, and name=N77), while "2015TN77" or any other letters in that first spot besides PDCX will (correctly) throw an error. The simplest fix seems to be to apply a validation test to the comet name to disallow comet names with numbers in the second position (but allow other non-alphabetic characters so names like d'Arrest don't also get screened out). I considered adding a validation test earlier on in the parsing to look for years followed by asteroid designations without spaces, but I feel like that has a better chance of introducing unintended consequences (especially once LSST takes us into the four-digit comet number era), whereas simply validating the name seems to address this specific issue without impacting any other functionality.

[mkelley]

Thanks!