Splitting up 3D regridding into lateral + vertical regridding

[NoraLoose]

For 3D fields, we are currently using xarray's .interp method for 3D regridding (which is inefficient). We want to split up 3D regridding into 2 steps:

• lateral regridding (which ultimately can be handled by xesmf, Handling xesmf dependency #133)
• vertical regridding

The vertical regridding is non-trivial since we have a spatially varying terrain-following target coordinate. We have two options:

1. Write our own routine.
2. Adapt xgcm.transform() to spatially varying target coordinates.

I'm in the process of pursuing 2. here: xgcm/xgcm#642