Change license mpl2

[nfahlgren]

Describe your changes
Starting with v4, replaces the MIT license with a Mozilla Public License Version 2.0. The reasons are summarized by Janis Lesinskis here: https://www.lesinskis.com/MIT-licence-community-hostile.html. Our main purpose here is to avoid the patent issues the post brings up. We also want to continue encourage commercial use and reuse/remixing of PlantCV, which is still allowable under MPLv2. But MPLv2 encourages contribution back to the original open source, while allowing for closed source additions on top.

Type of update
Is this a: update

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[codecov]

Codecov Report

Merging #1334 (304fcd2) into release-4.0 (8d9d7c3) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release-4.0     #1334   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              156       156           
  Lines             6829      6829           
=============================================
  Hits              6829      6829           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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