Add ability to return arbitrary list of variables

[alexanderst]

Now there's no ability to define method signatures which is array with unknown number of elements.
For example, to generate such method as below we need to specify expected number of outputs, though we don't know it.
SDVariable[] meshgird(SDVariable[] inputs)

[alexanderst]

It is decided to not touch outputs for now. The only requested change is to add ability to generate varags for input types, the goal is to generate this signature:
SDVariable[] meshgrid(boolean arg, SDVariable... inputs)