Tests for plotting functions and methods, other increases in test coverage

[ulupo]

Reference issues/PRs Addresses several remaining gaps in coverage (but still not the ones indicated in #182).

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Description

• Adds tests for all PlotterMixins or other classes containing plot methods. These are just dummy tests implementing trivial calls to fit_transform_plot or plot.
• Updates .coveragerc file so that gtda.images is included in coverage reports, gtda.utils._docs and gtda.utils.intervals are excluded, and all test directories are excluded.
• Adds simple tests for Filtering.
• Adds simple shape tests for BettiCurve and PersistenceLandscape.
• Minor typo fixes and other code style improvements.

Checklist

• [x] I have read the guidelines for contributing.
• [x] My code follows the code style of this project. I used flake8 to check my Python changes.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed. I used pytest to check this on Python tests.

[ulupo]

@wreise @gtauzin one downside of having plot methods which not only create figures but also show them is that locally running tests causes one tab per plot to be automatically opened by my browser. Have you encountered/resolved this?

[ulupo]

The failures in the manylinux builds seem to be related to a problem with the boost website. I wouldn't worry too much about it yet, hopefully it will be up and running soon.

[ulupo]

@gtauzin this is ready to merge and the new coverage percentage is between 93% and 94%. Due to issues at the boost website's end (Forbidden errors when following the tar.gz download link), we are delaying merging until the latter works as expected again.