Set up a magnetic wind problem: hard-coded for model mwAz4-xxx-HLLD
Hard-code for problem wmCz8-xxx-HLL
Add a current loop function and a models module to Mara

The current loop function does not do the finite differencing from
vector potential correctly yet.
Fix the current loop function, vector potential is differenced correctly
Improve robustness of current loop code
Set up wind-rmhd problem with runtime parameters
Change the wind-rmhd ramp profile to r * exp
Fix splash screen to show GIT_SHA correctly
Fiddle some more with the wind source parameters

This method might have to get left behind, the next commit should
introduce a source term meant to drive the interior solution toward
something known and fixed.
Merge branch 'wind'
Add updated quartic solver, does not change symmetry issues