CREST Basic I/O

[arepstein]

Summary

• Added CRESTOutput to parse CREST output directories in pymatgen.io.xtb.outputs
• Added CRESTInput to write CREST input files in pymatgen.io.xtb.inputs
• Added tests and test files for CRESTOutput and CRESTInput

Additional dependencies introduced (if any)

• None

TODO (if any)

• XTB i/o still missing
• CRESTOutput supports the default conformer searching method, but needs tests for non-default methods.
• CRESTInput supports writing a coordinate file and a .constrains file, but other files might be valuable to add
• Add parsing of .xcontrol
• Add improved error handling methods

Checklist

Work-in-progress pull requests are encouraged, but please put [WIP]
in the pull request title.

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• [ x] Code is in the standard Python style.
  Run pycodestyle and flake8
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• [x ] Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Type annotations are highly encouraged. Run mypy
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• [ x] Tests have been added for any new functionality or bug fixes.
• [ x] All existing tests pass.

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[coveralls]

Coverage decreased (-0.6%) to 83.093% when pulling ba54f14 on arepstein:ae_Crest into 14150a9 on materialsproject:master.

[mkhorton]

I'm going to merge this since the only issue is the same listing issue affecting the main branch and otherwise this PR looks excellent -- thank you @arepstein !