bundle `calc_*` functions

The `calc_*` functions all seem very related (they are all trying to calculate some form of climate index/variable, right?).

Would it make sense to:
1) bundle them into a single .R script
2) link their documentation in the same .Rd?

@kdaust and @cmahony

Start by locating the calc_* functions and their existing documentation in the climr package. Review how related climate indices and variables are currently organized, then determine whether consolidating the functions and linking their documentation is appropriate; done means the organization and documentation are consistently updated.

Written by the indexing model from the issue text.