Implement Jacobian determinant

[sgdecker]

Description Of Changes

Implements an equivalent to GEMPAK's JCBN function. The approach is modeled on the deformation calculations.

This is a draft for now because:

1. I still need to do testing
2. I am unsure about the check_units decorator. I imagine the common scenario is to do J(u,v), but really the two scalar quantities can be anything, so perhaps I should use:

@check_units(dx='[length]', dy='[length]')

instead? And does this have any implications for the "Units?" column in the GEMPAK Comparison Guide?

Checklist

• Closes Implement Jacobian determinant #2356
• Tests added
• Fully documented

[dopplershift]

Strictly speaking, vector_derivative assumes (for purposes of math with x/y-based vectors on a Cartesian plane) that u and v are vector components. So I don't think this implementation will work for just any pair of scalars? Any idea what GEMPAK implementation needs?

[dopplershift]

Looks like GEMPAK assumes scalars, so we can do that too. Probably should be using geospatial_gradient() like advection() does--which matches the check_units() you wrote above too.