Finalize and document FunctionOperator constructor

[gaurav-arya]

The constructor is currently not documented, and accepted_kwargs is also a little awkward (inconsistent with its other usages too, since this one takes in both the fields and the values). We should revisit this soon and finalize the interface.

[vpuri3]

thanks

[gaurav-arya]

We should also handle #57 when doing this -- I think it's an important issue. But the current version looks good enough to start propagating through SciML, so we can leave this alone for a little bit