Cannot unit test mayavi features on travis

[jmp75]

Tried to have mayavi related features unit tested. OK on my machine with a graphical env, but not on travis.
For the record, but not a battle I intend to fight actively.

  Fatal Python error: Aborted
Current thread 0x00007f0ceef72740 (most recent call first):
  File "/home/travis/miniconda/envs/ela_test/lib/python3.7/site-packages/pyface/ui/qt4/init.py", line 40 in <module>
etc. etc.
 File "/home/travis/build/csiro-hydrogeology/pyela/ela/visual3d.py", line 3 in <module>

/home/travis/.travis/functions: line 104:  9312 Aborted                 (core dumped) coverage run -m pytest

The command "coverage run -m pytest" exited with 134.

[banesullivan]

You'll likely need to set up a headless display - we have a nifty helper to handle this on Travis from the PyVista project: https://github.com/pyvista/gl-ci-helpers

See the info in that repo for most up to date info

[banesullivan]

And ensure the VTK backend is rendering offscreen - I'm not sure how that is done with Mayavi...