Add `find_missing_groups` function

[pw0908]

Things left to do in this branch:

• [x] Add functionality to reduce the proposed set of groups to a minimal representation.
• [x] Add tests
• [ ] Add docs
• [ ] Maybe clean-up the way environmental smarts are handled? (For groups where we've combined multiple heavy atoms, only specify environments for the atom which is bonding to another group)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 95.53073% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 83.20%. Comparing base (91ed072) to head (6742ff7).

Files	Patch %	Lines
src/missing_groups.jl	95.77%	6 Missing :warning:
src/group_search.jl	94.59%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #5 +/- ## =========================================== + Coverage 60.00% 83.20% +23.20% =========================================== Files 5 6 +1 Lines 245 399 +154 =========================================== + Hits 147 332 +185 + Misses 98 67 -31 ```

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