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