Allow users to specify param type in cli or yaml for subset of numpy functions

[schuylerfried]

Would be pretty cool if you could do something like:
--delays 0 100 10 --delays_function Linspace
or
--delays 0 100 10 --delays_function Arange

and the corresponding Param would be created. You can imagine having a Param called NumpyFunctionParam that isn't specific and will add an extra x_function argument to the parser or to the json dictionary. That function will specify which function should be executed on those parameters.

This would be a really cool feature and actually wouldn't require that significant of a refactor (if we were to implement it the way I've specified above