Added measurement type to waterfall colorbar

[d-chambers]

Description

PR opened for @SeisMatt for his work on adding the meaning and units to the waterfall colorbar.

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

Merging #293 (f247669) into master (a2d2782) will increase coverage by 0.00%. Report is 5 commits behind head on master. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #293   +/-   ##
=======================================
  Coverage   99.28%   99.29%           
=======================================
  Files          83       83           
  Lines        6619     6626    +7     
=======================================
+ Hits         6572     6579    +7     
  Misses         47       47           

Flag	Coverage Δ
unittests	99.29% <100.00%> (+<0.01%)	:arrow_up:

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

Files	Coverage Δ
dascore/viz/waterfall.py	100.00% <100.00%> (ø)

... and 4 files with indirect coverage changes