Add the xTB backends to environment.yml

[galjos]

environment.yml already installed DFTB+ but predates the xTB engines (#68, #70). Add the xTB backends so conda env create -f environment.yml provisions every calculation engine in one command.

dependencies:
  - python>=3.12
  - dftbplus        # DFTB+ engine: `dftb+` and `modes`
  - xtb             # native xtb binary, for --engine xtb-cli
  - tblite-python   # GFN-xTB via tblite, for --engine xtb
  - pip
  - pip:
      - "-e .[test,lint]"

After conda env create -f environment.yml && conda activate thermoscreening, thermo doctor reports every backend as found (pairs with #71). README updated to say the env covers all backends.

Verified the three backends resolve on conda-forge (dftbplus-25.1 nompi, xtb-6.6.1, tblite-python-0.3.0); YAML validated.

This is the "installable through the project" follow-up: the compiled backends (DFTB+/modes/xtb/tblite) can't be pip-installed, so the conda env is how the project bundles them, while thermo setup-dftb handles the Slater-Koster and GBSA parameter files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.66%. Comparing base (b87b46c) to head (2448873).

Additional details and impacted files

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##             main      #72   +/-   ##
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  Coverage   96.66%   96.66%           
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  Files          24       24           
  Lines        1527     1527           
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  Hits         1476     1476           
  Misses         51       51           

Flag	Coverage Δ
unittests	96.66% <ø> (ø)

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