Unneeded warning from xarray dimension handling

[dopplershift]

The following code:

mpcalc.advection(isen_press, u=isen_u, v=isen_v)

produces a warning:

/Users/rmay/repos/metpy/src/metpy/xarray.py:1473: UserWarning: Vertical dimension number not found. Defaulting to (..., Z, Y, X) order.

when isen_press, isen_u, and isen_v are all 2D and matching shape. That seems unneeded for this case and probably a bit confusing to users. Not sure how much of a pain it would be to fix that though...

[dopplershift]

So the problem for advection is that the function takes vertical_dim, to support anyone wanting to do vertical advection. It falls out of the call when we go to calculate, but at the time the warning is made it's not known this isn't really needed.

That puts this squarely in the pile of "painful to fix".