[bugfix] Fixes OAP unit mismatch issue in OAPs. Fixes #1906

[chummels]

PR Summary

Issue #1906 demonstrates how OffAxisProjections in demeshening project with weird units. This PR fixes that issue by assuring that the path length unit is in the default length units for that dataset.

This script demonstrates this solution (note how the zlims and units are the same for each on-axis/off-axis pair now):

import yt
ds = yt.load('FIRE_M12i_ref11/snapshot_600.hdf5')
yt.ProjectionPlot(ds, 'z', ('gas', 'density')).save('on.png')
yt.ProjectionPlot(ds, 'z', ('gas', 'temperature'), weight_field=('gas', 'density')).save('on_temp.png')
yt.OffAxisProjectionPlot(ds, [0,0,1], ('gas', 'density'), north_vector=[0,1,0]).save('off.png')
yt.OffAxisProjectionPlot(ds, [0,0,1], ('gas', 'temperature'), weight_field=('gas', 'density'), north_vector=[0,1,0]).save('off_temp.png')

On Axis Projection in density:

Off Axis Projection in density (identical):

On Axis Projection in density-weighted temperature:

Off Axis Projection in density-weighted temperature:

[ngoldbaum]

Looks great, thanks for the fix Cameron!

[ngoldbaum]

The travis failure is just a timeout. Merging.