For WRR R1 review

Revised after peer review. Now updated to revision 1 of the manuscript.

Update with refactored mbr code.

The mbr package has just been refactored. It now contains only functions for the mass balance adjusted regression. Functions that are used for optimizing the site selection are removed and will be included in a future package. This repo is updated accordingly: the input selection functions that were inside mbr are now sourced direct in an R script.

For review

First release, for peer review.