Fix pickling for MSIDset and add tests

[taldcroft]

Description

This fixes a problem when unpickling an MSIDset and adds tests for both that class and MSID.

Testing

• Passes unit tests on MacOS
• [n/a] Functional testing

[jeanconn]

It wasn't clear what the original problem was to me from the description, so I ran the new tests on the old code to see

>       dat2 = pickle.loads(pickle.dumps(dat))
E       TypeError: __init__() missing 1 required positional argument: 'msids'

Ska/engarchive/tests/test_fetch.py:91: TypeError

And I see that this goes away with the bugfix.