NPP integration

[samaperrin]

Why have changes been made?

@ivarherfindal wrote a script for downloading net primary productivity using mods data. We have integrated this into the workflow.

What changes have been made?

• data/external/focalCovariates.csv - NPP is added to the covariate list, with 'modis' as an external source
• data/external/get_modis.R - a get_modis function now pulls in the necessary data
• pipeline/import/utils/defineEnvSource.R - modis data now added in to our list of covariates to import
• pipeline/import/environmentalImport.R - folder naming rearranged