Adds support for iterable number args

Supports arguments that are __iter__ objects filled with Number-like elements (includes numpy arrays, lists, tuples, sets)
Reworks args to *Function; improves graphs

Adds arg_list to NumexprFunction and moves the arg_list and param_map into AbstractFunction for better code reuse - also moves get_function_map to AbstractFunction, used by both subclasses

Improved graph output - includes argument names for parent functions so that lineage is more apparent.  Graphs can be generated without a param_map, arguments are marked as "missing"
Merge pull request #26 from blazetopher/pfunc_updates

Updates to Parameter Functions