Simplify data_frame/call framework

[Michael Schubert]

Right now, we have the following functions:

df$expand_grid           # creates data.frame that can not be used in df$call()
df$create_index          # creates IndexedCall object
df$create_formula_index  # creates IndexedFormula object
df$call                  # needs an IndexedCall object

Creating an index and doing a call should be more easily accessible. This includes:

• Do the index creation and call in one step, e.g. fun(fun, iter, const, expand_grid, hpc_args)
• Be able to use call() on any data.frame where the rows should be function arguments

[Michael Schubert]

deprecated/won't fix