Add default fallbacks for head and vexity with MOI sets

[odow]

Closes #630

One possible solution to discuss.

• The head method is reasonable.
• The vexity method is up for debate. There are arguments for and against.

[codecov[bot]]

Codecov Report

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

Project coverage is 97.86%. Comparing base (de4bff2) to head (4f004be). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #632 +/- ## ========================================== + Coverage 97.82% 97.86% +0.04% ========================================== Files 88 88 Lines 5141 5099 -42 ========================================== - Hits 5029 4990 -39 + Misses 112 109 -3 ```

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[ericphanson]

The default vexities worry me :/