* [MRG] Create a new figure and test each plot type #127
- move `plot()` to `plotting.py` as `plot_pdf()`
- modify plotting functions to return matplotlib figures
- add `test_plotting.py` and baseline images
- import `plot_pdf()` in `__init__`
- update `cli.py` to use `plot_pdf()`
- update advanced usage docs to reflect changes
* Change matplotlib backend for image comparison tests
* Update plotting and tests
- use matplotlib rectangle instead of `cv2.rectangle` in
`plot_contour()`
- set matplotlib backend in `tests/__init__`
- update contour plot baseline image
- update `test_plotting` with more checks
* Update plot tests and config
- remove unnecessary asserts
- update setup.cfg and makefile with `--mpl`
* Add to
* Add tolerance
* remove text from baseline plots
update plot tests with `remove_text`
* Change method name, update docs and add pep8
* Update docs
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