Modified checkpoint handler to store best model

[pbenner]

Fixes #127

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% :tada:

Comparison is base (21e452c) 76.10% compared to head (20f9793) 76.14%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #129 +/- ## =========================================== + Coverage 76.10% 76.14% +0.03% =========================================== Files 23 23 Lines 3604 3609 +5 =========================================== + Hits 2743 2748 +5 Misses 861 861 ``` | [Files Changed](https://app.codecov.io/gh/usnistgov/alignn/pull/129?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None) | Coverage Δ | | |---|---|---| | [alignn/train.py](https://app.codecov.io/gh/usnistgov/alignn/pull/129?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=None#diff-YWxpZ25uL3RyYWluLnB5) | `86.12% <100.00%> (+0.13%)` | :arrow_up: |

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[knc6]

Hi @pbenner

Thank you for the PR. In order to keep the backward compatibility of the package, we should have two handlers: 1) using the trainer as well as 2) use your idea for saving on evaluator so that both last two steps and best evaluator models are saved. Are you open to adding this to your PR?

[pbenner]

Dear Kamal,

Sure, that's a good idea. I've just pushed the implementation to my branch.

Best, Philipp