BUG: constrain variable probes orthogonal

[carterbox]

Purpose

Adding some assertions showed that the probe orthogonalization for variable probes was not correct because the resulting probes were not sorted by power from most to least.

Approach

A new parameter for the orthogonalize_gs method allows the correct probe orthogonalization for these probes and a sorting step is added to sort the probes by their power.

Pre-Merge Checklists

Submitter

• [x] Write a helpfully descriptive pull request title.
• [x] Organize changes into logically grouped commits with descriptive commit messages.
• [x] Document all new functions.
• [ ] Click 'details' on the readthedocs check to view the updated docs.
• [x] Write tests for new functions or explain why they are not needed.
• [x] Address any complaints from pep8speaks.

Reviewer

• [ ] Actually read all of the code.
• [ ] Run the new code yourself; the included tests should make this easy.
• [ ] Write a summary of the changes as you understand them.
• [ ] Thank the submitter.

[carterbox]

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