Any thoughts on an interpolate function in Function which would
interpolate coefficients of a sub function only? At the moment, I have a
solver in which I can't use SpecialFunction::IsOutFlowFacet because the
velocity is a sub function of the solution Function. I can use
SubFunction, but SubFunction makes a copy of the vector every time. I
can think of a number of other examples in which this would be useful.
An alternative would be to have a variant on a SubFunction which shares
the vector of the Function it is a part of.
