TESS extended mission cubing updates

[ceb8]

This pull request encompasses the following:

• Allow for cubes to be made with less memory than needed to hold the entire cube in memory
  Available memory is not an input argument and defaults to 40GB.
• Improved testing for all functionality
• Additional small tweaks to new tox bases CI
• Addresses Precision on timestamps in Tess data #18

[codecov]

Codecov Report

Merging #35 into master will increase coverage by 10.09%.
The diff coverage is 98.69%.

@@             Coverage Diff             @@
##           master      #35       +/-   ##
===========================================
+ Coverage   86.75%   96.84%   +10.09%     
===========================================
  Files           6        6               
  Lines         770      825       +55     
===========================================
+ Hits          668      799      +131     
+ Misses        102       26       -76     

Impacted Files	Coverage Δ
astrocut/make_cube.py	98.78% <98.58%> (+6.63%)	⬆️
astrocut/cube_cut.py	99.36% <100.00%> (+7.32%)	⬆️
astrocut/cutouts.py	94.48% <100.00%> (+16.01%)	⬆️

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