Lazy evaluation on avail error list

[cmleinz]

Effects

Lazily evaluate the avail error list to keep the happy path free of unnecessary allocations.

If this is a new feature or a bug fix ...

• Yes, the branch I'm proposing to merge is called issue-xyz where xyz is the number of the issue.

If this change adds or modifies a validation case

• No.
• Yes and:
  • I have coded up, or updated, an existing test case; and
  • I have provided a GMAT script which confirms the result(s); and
  • I have updated the VALIDATION.md file with the new error data between nyx and GMAT.
  • I will specify the version of GMAT used for the validation.

[ChristopherRabotin]

Looks good, thanks! I'll be starting to integrate ANISE in Nyx shortly, but that will likely take several weeks.

I'm a bit surprised about the docker error in the packaging step of python. I suspect that's because you don't own the repo, so I'll ignore that issue and expect it to resolve itself.