dft to be idempotent for already transformed coords

[d-chambers]

Description

closes #395

This PR makes dft idempotent, meaning calling dft on the same dimension twice does nothing. See #395 for motivation and example.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes.
• documented the new feature with docstrings or appropriate doc page.
• included a test. See testing guidelines.
• your name has been added to the contributors page (docs/contributors.md).
• added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.83%. Comparing base (dea01ab) to head (c626e35).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #397   +/-   ##
=======================================
  Coverage   99.83%   99.83%           
=======================================
  Files         106      106           
  Lines        8625     8640   +15     
=======================================
+ Hits         8611     8626   +15     
  Misses         14       14           

Flag	Coverage Δ
unittests	99.83% <100.00%> (+<0.01%)	⬆️

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[d-chambers]

also fixes #390