close matplotlib figures in notebook_vars; enable testing notebook_vars; add plotting logic to notebook_vars test notebook

[AgnieszkaZaba]

No description provided.

[AgnieszkaZaba]

Running smoke tests in PySDM, notebook_vars is executed multiple times within one test-suite.
It cause this RuntimeWarning

RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface 
(`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. 
(To control this warning, see the rcParam `figure.max_open_warning`). Consider using `matplotlib.pyplot.close()`.

[AgnieszkaZaba]

Triggered warning in open-atmos/PySDM#1763, when updating test-suites.

[slayoo]

this looks like a possible source of unintended side effect, would doing .close() instead of .clear() when replacing show_plot() above work?