MRG, MAINT: Simpler vector params

[larsoner]

We can avoid a lot of gymnastics having to do with maximum magnitudes and even scalars by setting the scale_mode='vector' in the quiver3d call.

[larsoner]

Output unchanged:

https://344-1551431-gh.circle-artifacts.com/0/html/auto_examples/plot_vector_meg_inverse_solution.html#sphx-glr-auto-examples-plot-vector-meg-inverse-solution-py

[codecov]

Codecov Report

Merging #291 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #291      +/-   ##
==========================================
- Coverage   75.06%   75.02%   -0.04%     
==========================================
  Files           7        7              
  Lines        2635     2631       -4     
  Branches      515      514       -1     
==========================================
- Hits         1978     1974       -4     
  Misses        479      479              
  Partials      178      178              

[larsoner]

Pushed a commit that works around Mayavi/VTK auto-scaling stuff

[larsoner]

Okay, done for now I think. I tweaked the scaling such that it's more robust to data changes

[larsoner]

We are due for a release, so I'll merge this then push out a release unless people object (I think I asked about this already a week or two ago but don't remember...?)