Alpine-3D netCDF time invariant fields

[EricKeenan]

We should add the following time-invariant fields to Alpine-3D netCDF output:

1. Latitude/longitude of each grid cell in WGS-84 (in addition to X and Y coordinates in EPSG 3031 which Alpine-3D already writes)
2. Grid cell area (m^2)
3. Grid cell elevation (m)