sped up average missing data

[mattjones315]

Sped up the function cas.solver.missing_data_methods.assign_missing_average, especially for ambiguous data, by a factor of ~100x. Makes a big difference when there's quite a lot of missing data.

[codecov[bot]]

Codecov Report

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

Comparison is base (0f7c330) 79.53% compared to head (81bf0ee) 79.61%. Report is 1 commits behind head on master.

:exclamation: Current head 81bf0ee differs from pull request most recent head 8e8cab1. Consider uploading reports for the commit 8e8cab1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #235 +/- ## ========================================== + Coverage 79.53% 79.61% +0.07% ========================================== Files 89 89 Lines 8064 8061 -3 ========================================== + Hits 6414 6418 +4 + Misses 1650 1643 -7 ```

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