Init eta checkpoint

[amoodie]

Rework the integration of these new fields into the checkpointing.

My prev PR (#281) was not thoroughly considered, and therefore was a breaking change, when I didn't mean for it to be. This PR adds handling and a warning when these fields are not found in a checkpoint file, but allows the loading to proceed.

Also fixes the broken documentation, because that checkpoint file was not working.

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 78.92%. Comparing base (27d0f45) to head (23bc8cf).
Report is 3 commits behind head on develop.

Files	Patch %	Lines
pyDeltaRCM/init_tools.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #283      +/-   ##
===========================================
+ Coverage    78.88%   78.92%   +0.04%     
===========================================
  Files           12       12              
  Lines         2619     2634      +15     
===========================================
+ Hits          2066     2079      +13     
- Misses         553      555       +2     

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