Tests for the GCMC and Widom classes.

[hdsassnick]

Pull Request

Description 📝

This PR merely adds a test for the Widom and the GCMCclass, likewise to the TMMC class. With those tests the overall test coverage increases to 58%.

Changes Proposed 🔧

Related Issues 📎

Fixes # <issue_number>
Closes # <issue_number>

Before submitting 📌

• I've read and followed all steps in the Making a pull request
  section of the CONTRIBUTING docs.
• I've updated or added any relevant docstrings following the syntax described in the
  Writing docstrings section of the CONTRIBUTING docs.
• If this PR fixes a bug, I've added a test that will fail without my fix.
• If this PR adds a new feature, I've added tests that sufficiently cover my new functionality.

Testing Instructions 🧪

Additional Notes 💬