Feather files

[AaronDJohnson]

Adds the ability to store Pulsar objects and noise dictionaries into FeatherPulsar objects which can be saved and read using pyarrow feather types.

Additionally, this

• Removes libstempo as a required dependency
• Removes PINT as a required dependency
• Removes to_pickle from BasePulsar: pickles should be removed and discouraged for many reasons
• Adds to_feather to BasePulsar
• Creates a new FeatherPulsar type which holds data and allows for reading/writing Pulsar objects
• Removes to_pickle test and adds to_feather test

[AaronDJohnson]

An important design choice is left to be made: FeatherPulsar could potentially be absorbed into BasePulsar. This would require a minimal amount of work, but may make the feather objects a little bigger than they are currently due to storage of _isort and _iisort and other pieces which are not currently stored.

[AaronDJohnson]

Tests are failing because of an installation issue with fastshermanmorrison

[codecov]

Codecov Report

Attention: Patch coverage is 70.76923% with 19 lines in your changes missing coverage. Please review.

Project coverage is 71.70%. Comparing base (4f6154a) to head (6006e18).
Report is 4 commits behind head on dev.

Files with missing lines	Patch %	Lines
enterprise/pulsar.py	70.76%	19 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              dev     #390       +/-   ##
===========================================
- Coverage   85.33%   71.70%   -13.64%     
===========================================
  Files          13       13               
  Lines        3158     3220       +62     
===========================================
- Hits         2695     2309      -386     
- Misses        463      911      +448     

Files with missing lines	Coverage Δ
enterprise/signals/white_signals.py	98.61% <ø> (ø)
enterprise/pulsar.py	28.77% <70.76%> (-65.03%)	⬇️

... and 2 files with indirect coverage changes

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