Initial draft of driver-weighted PCA

[voglerdaniel]

Addresses #349.

Next steps (can run in parallel):

• We need to create an index structure file - a two-column .csv table that maps each driver to each indicator in the Water Reuse index
• Also need to add functionality to the creator script that automatically produces this mapping and delivers it as an asset
• Someone should test this for initial index creation - source driver_weighted_pca.R and run multidriver_pca() to create an initial Water Reuse index
• Finally, multidriver_pca() needs to be integrated into the R script that runs when the user clicks SUBMIT in the web interface