Add parameter descriptions in slimr_template

[rdinnager]

Add an optional description argument to slimr_template along with a function to print out a table of all templated variables in a simulation template, including the description. The description won't affect the simulation at all but will be stored in case you forget what a variable is supposed to do when coming back to a simulation after awhile.

[rdinnager]

While we are at, should add a range argument to specify range of allowed values. Then an error can be thrown if a value out of the expected range is provided (perhaps by accident). We can also use this to generate automatic shiny inputs for the shiny app generator function planned.