Don't gen xyz when checking if a species is monoatomic

[alongd]

To avoid inf. recursions (is_monoatomic → get_xyz → get_cheap_conformer → is_monoatomic), don't generate a cheap xyz when checking if a species is monoatomic

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 74.49%. Comparing base (155f5af) to head (0cac468).

Files with missing lines	Patch %	Lines
arc/species/species.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #785      +/-   ##
==========================================
- Coverage   74.50%   74.49%   -0.01%     
==========================================
  Files         103      103              
  Lines       28114    28117       +3     
  Branches     5827     5828       +1     
==========================================
+ Hits        20946    20947       +1     
- Misses       5678     5679       +1     
- Partials     1490     1491       +1     

Flag	Coverage Δ
unittests	74.49% <66.66%> (-0.01%)	⬇️

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