Use PROJ_NETWORK setting appropriately

[emlys]

PROJ version 7 added the option to download grids from a CDN, as an alternative to storing grids locally. These grids give better accuracy? to reprojections. The option is configured with the env variable PROJ_NETWORK=ON. It's set to ON by default in a conda environment (I guess when gdal is installed?)

• Is there any invest functionality that doesn't work without the grids?
• If so, should we include the grids in our distribution so that a network connection isn't required?
• Is there any invest functionality that would be faster or more accurate using the grids?
• Should invest set the PROJ_NETWORK variable? Rather than falling back to its setting in the user's environment, which they might not be aware of
• Should we mention the PROJ_NETWORK variable in invest documentation?