Use eigh in bath op assertion

[piperfw]

#165

The constructor already asserts the coupling operator is Hermitian, so using eigh should be safe and not fail in the way described in the issue.

Never done a one-character Pull Request before. Ran the tests just to be sure!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.12%. Comparing base (0015fc2) to head (434b047).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #166   +/-   ##
=======================================
  Coverage   92.12%   92.12%           
=======================================
  Files          24       24           
  Lines        4572     4572           
=======================================
  Hits         4212     4212           
  Misses        360      360           

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[piperfw]

If we wanted to be more thorough, we could write some tests using an operator such as in the issue example