Fix xtb opt with cartesian coordinates

[t-young31]

Resolves #245

Fixes cartesian coordinate constraints in XTB optimisations. I'm not convinced this ever worked correctly!

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Checklist

• [x] The changes include an associated explanation of how/why
• [x] Test pass
• [x] Changelog has been updated

[codecov[bot]]

Codecov Report

Merging #247 (5f3151a) into v1.3.5 (0d03666) will increase coverage by 0.00%. The diff coverage is 100.00%.

:exclamation: Current head 5f3151a differs from pull request most recent head 53ee7b6. Consider uploading reports for the commit 53ee7b6 to get more accurate results

@@           Coverage Diff           @@
##           v1.3.5     #247   +/-   ##
=======================================
  Coverage   97.14%   97.15%           
=======================================
  Files         196      196           
  Lines       20172    20183   +11     
=======================================
+ Hits        19597    19608   +11     
  Misses        575      575           

Flag	Coverage Δ
unittests	97.15% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
autode/wrappers/XTB.py	98.53% <100.00%> (-0.03%)	:arrow_down:
tests/test_wrappers/test_xtb.py	99.47% <100.00%> (+0.04%)	:arrow_up:

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