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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #397 +/- ## ======================================= Coverage 99.83% 99.83% ======================================= Files 106 106 Lines 8625 8640 +15 ======================================= + Hits 8611 8626 +15 Misses 14 14 ``` | [Flag](https://app.codecov.io/gh/DASDAE/dascore/pull/397/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/DASDAE/dascore/pull/397/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | `99.83% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE#carryforward-flags-in-the-pull-request-comment) to find out more.

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[d-chambers]

also fixes #390