Simpler function name alfam2() and small improvements

The model function can be called with alfam2() instead of the original ALFAM2mod() now (although the original name is still an alias, and so still works as before). This version also includes some new checks to input arguments and associated warning messages, that should make use easier.

You can install this release directly within R with

devtools::install_github('sashahafner/ALFAM2@v2.0')

To update (if you already have ALFAM2 installed), remove the older version first with this

remove.packages('ALFAM2')