Extend tests for utopya.plot_funcs module

With the broad addition of additional plot functions, utopya test coverage dropped to:

This is mostly due to the utopya.plot_funcs module not being thoroughly tested. While it is hard to visually test them, we should at least have coverage tests of these plot functions...

Proposal

• Extend test_plotting.py to include more cases.
• Find a way to visually inspect the resulting plots:
  • In the testtools.py module, where ModelTest is defined, make it possible to check for an environment variable to determine whether to use_tmpdir or not
  • In the CI, set the corresponding environment variable on the test_utopya job
  • In that job, configure artifacts to include utopia_output (might have to set that to within the repo ...)