Implement caching of SVO filters

[cgarling]

Implements caching of filters queried from SVO. Includes an update_filters() function that will redownload all cached filters from SVO, effectively refreshing the cache.

@lucasvalenzuela how does this look?

[codecov]

Codecov Report

❌ Patch coverage is 36.36364% with 28 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.76%. Comparing base (21e9344) to head (a5904b2).

Files with missing lines	Patch %	Lines
src/svo.jl	36.58%	26 Missing ⚠️
src/plots.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   80.60%   76.76%   -3.84%     
==========================================
  Files           6        6              
  Lines         366      396      +30     
==========================================
+ Hits          295      304       +9     
- Misses         71       92      +21     

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[lucasvalenzuela]

I would save these as .xml files instead

[lucasvalenzuela]

It works really well and looks good to me! I have added one comment in the review (xml vs. txt files) and also added a PR adding clear_filters to remove all filters in the cache: cgarling#7

One thing one could still add is a keyword argument to get_filter that forces the downloading of the file, and/or a keyword argument that prevents the caching of the downloaded file. And one could equivalently add an argument to clear_filters or a new clear_filter to remove one single specified filter. But maybe that is too much of an edge case.