Feature Request: Have list_vars return a list of the variables

[billdenney]

Having list_vars return a human-usable list of variables is helpful. But in times when I'm reporting on many models simultaneously, it would help to be able to check the var list to see if something is defined. As an example, I'd like to be able to do something like the following in my code:

if (length(list_vars(my_xpdb)$ipred) == 0) {
  my_xpdb <- set_var_types(my_xpdb, ipred="CONC")
}

[bguiastr]

related to #124

[billdenney]

Whoops, yes they are the same.