added option to plot at visual center of fields

[bobmyhill]

Description

This PR adds the option to plot field labels at the "visual center" of a field, rather than the centroid. The "visual center" is defined as the center of the largest inscribed circle. An option is provided to pass a vertical exaggeration, which instead finds the center of the largest inscribed ellipse.

Builds on #96.

Motivation and Context

For fields with reflex boundary vertices ("concave polygons"), the centroid can plot close to a boundary or even outside the field. The "visual center" of a polygon is often a better location for field labels.

@morganjwilliams: I'm not sure whether this is something you want in the code, but I needed it anyway, so thought I'd open the PR.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[morganjwilliams]

@bobmyhill given this is adding a dependency, I might see whether it's simpler to just add the relevant code to pyrolite (it looks like the package is just one file, has no dependencies and is MIT licensed...).

[coveralls]

Pull Request Test Coverage Report for Build 6510161844

• 130 of 146 (89.04%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.009%) to 90.42%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pyrolite/plot/spider.py	1	2	50.0%
pyrolite/util/skl/transform.py	0	2	0.0%
pyrolite/util/plot/center.py	113	126	89.68%

Totals
Change from base Build 6505996614:	0.009%
Covered Lines:	6107
Relevant Lines:	6754

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💛 - Coveralls

[bobmyhill]

@morganjwilliams good point, I've added the code with a reference to the original repo and removed the new dependency. I've also added to the test suite.

[morganjwilliams]

Great, thanks @bobmyhill! Will merge it across.