Add unit tests for iqcalc with mock_sky module

[ejeschke]

Adds more unit tests for iqcalc module via a small utility module called mock_sky.

[ejeschke]

Whups, need to add scipy in the CI builds in order to add this. Alternatively, could test for scipy and skip tests for which it is needed when it is not installed. Hmmm...

[pllim]

Is this the unit test you mentioned in #769 (comment) ?

[pllim]

@ejeschke , I feel like this is duplicating what is already in photutils; see https://photutils.readthedocs.io/en/stable/datasets.html .

[ejeschke]

@pllim, good catch, I'm not super familiar with photutils, but it looks like it can generate good sources for testing iqcalc. Let me try modifying this PR to use it.

[ejeschke]

Is this the unit test you mentioned in #769 (comment) ?

Yep

[ejeschke]

Needs refactor to use photutils.