Add proximity-based metrics for irreplaceability

[jeffreyhanson]

This paper outlines two new metrics for calculating evaluating irreplaceability (https://doi.org/10.1111/cobi.13806), and this new R package provides code to calculate them (https://cran.r-project.org/package=proxirr). It would be great to update prioritizr so that users can easily calculate these metrics to evaluate their solutions.