Convenience function that installs all extra packages used in ENMTools. ENMTools uses
functions from a lot of external packages, and due to CRAN best practices it doesn't install
those packages by default. The function install.extras() just calls install.packages with a
list of all of the extra packages. At present this list includes mgcv, ecospat, randomForest,
hypervolume, ape, leaflet, ranger, CalibratR, caret, and ResourceSelection.