Add CastAndCallScalarField and CastAndCallVecField to NewFilterField
These methods somewhat simplify doing the CastAndCall from an
UnknownArrayhandle that comes from an input field.
The motivation for this change is for the User's guide, where I am
having a chicken-and-egg problem of wanting to describe how to make a
simple filter implementation without having to go into details about
UnknownArrayHandle.
Also updated the CrossProduct filter to the new filter structure,
which provides a demonstration of CastAndCallVecField.