Reduce peak memory use by not reading full file to get RA DEC columns

[jeanconn]

Description

Reduce peak memory use by reading just the RA and DEC fields to get them (used for indexing into the agasc).

Interface impacts

Testing

Unit tests

• Mac

Independent check of unit tests by @taldcroft:

• Mac

Functional tests

I looked at this before and after with memory profiler output and confirmed that the change means that, using the proseco agasc file as an example, only the 60Mb columns are read into memory instead of the file (which peaks more at ~600Mb).

[jeanconn]

I considered updating the code to remove the explicit cache, and provide an option to not-cache the RA DECs altogether... but maybe it makes sense to limit scope creep.

[taldcroft]

Looks good!

[taldcroft]

And even though I'm now fully convinced that returning from within a context block will run the __exit__ method (https://stackoverflow.com/questions/9885217/in-python-if-i-return-inside-a-with-block-will-the-file-still-close), I still think it smells bad. (i.e. it has a popular a stack overflow answer so it is confusing).