Improve unit test coverage

[medley56]

I noticed that we had low-ish coverage on a couple modules, mostly on the error-handling since most of the tests are run correctly. I found some locations where we were checking for exception cases in evaluate methods when we should have been pre-checking in either init or from_xml.

I added some additional coverage to the packets.py module but I'm thinking that will probably need rework after @greglucas 's refactoring in #140

Checklist

• Changes are fully implemented without dangling issues or TODO items
• Deprecated/superseded code is removed or marked with deprecation warning
• Current dependencies have been properly specified and old dependencies removed
• New code/functionality has accompanying tests and any old tests have been updated to match any new assumptions
• [NA] The changelog.md has been updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.43%. Comparing base (bc05878) to head (9cee12e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
+ Coverage   91.89%   93.43%   +1.54%     
==========================================
  Files          36       37       +1     
  Lines        2553     2605      +52     
==========================================
+ Hits         2346     2434      +88     
+ Misses        207      171      -36     

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[greglucas]

Yeah, one of us will have to fix some merge conflicts, but overall this doesn't seem like it will cause too much headache CCSDSPacket -> Packet mostly.