eta_init and eta0 in checkpointing

[amoodie]

Adds eta_init and eta0 to the output of checkpoints.

Also adds these to checkpoint loading and adds a simple test to check that all fields are present. This is just to try and promote consistency.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 78.88%. Comparing base (459c299) to head (27d0f45).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #281 +/- ## =========================================== + Coverage 78.86% 78.88% +0.02% =========================================== Files 12 12 Lines 2616 2619 +3 =========================================== + Hits 2063 2066 +3 Misses 553 553 ```

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