Ft transforms

[d-chambers]

Description

This PR:

• Implements a new transform module called fourier for performing the discrete Fourier transforms (dft) and its inverse.
• Adds a new section to the docs called "Notes" and a note on dft scaling.
• Adds a few convenience methods to the coord manager and patch

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

Merging #192 (68b7fc7) into master (2aabaa2) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   98.25%   98.33%   +0.08%     
==========================================
  Files          66       67       +1     
  Lines        5208     5353     +145     
==========================================
+ Hits         5117     5264     +147     
+ Misses         91       89       -2     

Flag	Coverage Δ
unittests	98.33% <100.00%> (+0.08%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
dascore/constants.py	100.00% <ø> (ø)
dascore/core/coordmanager.py	100.00% <100.00%> (ø)
dascore/core/patch.py	100.00% <100.00%> (ø)
dascore/core/schema.py	100.00% <100.00%> (ø)
dascore/examples.py	100.00% <100.00%> (ø)
dascore/proc/__init__.py	100.00% <100.00%> (ø)
dascore/proc/basic.py	100.00% <100.00%> (ø)
dascore/transform/__init__.py	100.00% <100.00%> (ø)
dascore/transform/fft.py	100.00% <100.00%> (ø)
dascore/transform/fourier.py	100.00% <100.00%> (ø)
... and 6 more

... and 1 file with indirect coverage changes

[d-chambers]

I am going to merge this for now so that the new docs build, then solicit feedback on the notes and method interface, then open a subsequent PR (if needed) to make any changes.