[MRG+1] SARIMAX only

[tgsmith61591]

Description

This fixes #211 by updating our ARIMA class to only use the statsmodels SARIMAX under the hood, per the thread here: statsmodels/statsmodels#6225 (comment)

It should not cause any breaking changes, though it will yield very slightly different results for some parameter combinations. It also will yield two new warnings for explicitly setting the old default values to some hyper-parameters.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (only in the sense that existing models may yield very slight differences in parameters)
• Documentation change

How Has This Been Tested?

All existing tests pass (with minor updates where required), and parity with R package functionality is the same as before.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes