More housekeeping

[esoteric-ephemera]

• To avoid "URI too long" type errors and convey more meaningful info back to the user, raise an error if >9 elements are queried (max num from testing that works)
• Remove dead code around molecules (unused resters + features that don't work because API endpoints haven't been deployed)
• Modify find_structure to improve performance and shift compute to user

[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.10345% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.64%. Comparing base (2d0ffff) to head (e47c371).

Files with missing lines	Patch %	Lines
mp_api/client/routes/materials/materials.py	95.23%	1 Missing ⚠️
mp_api/client/routes/molecules/summary.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1068       +/-   ##
===========================================
+ Coverage   71.96%   83.64%   +11.67%     
===========================================
  Files          55       47        -8     
  Lines        3082     2672      -410     
===========================================
+ Hits         2218     2235       +17     
+ Misses        864      437      -427     

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