Allow users to specify a custom cache directory

[nksaunders]

It would be useful to be able to specify a custom directory for the downloaded TESScut fits files. This PR adds a keyword argument to Source to input a custom cache directory.

To do before merging:

• disambiguate download_dir and fn_dir, and make sure they don't confict
• make sure TargetData knows about the right cache directory for the input Source data

[codecov]

Codecov Report

Merging #135 into master will increase coverage by 0.04%.
The diff coverage is 78.26%.

@@            Coverage Diff             @@
##           master     #135      +/-   ##
==========================================
+ Coverage   45.08%   45.12%   +0.04%     
==========================================
  Files          18       18              
  Lines        1719     1724       +5     
==========================================
+ Hits          775      778       +3     
- Misses        944      946       +2

Impacted Files	Coverage Δ
eleanor/source.py	53.46% <78.26%> (+0.16%)	⬆️

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