Replace function_value_<N>d with something more general

[marcinlos]

While these classes do the job and are widely used in the example codes, they are more of a quick hack than a proper solution. They should be replaced with a template struct offering a more complete interface (e.g. multiplication of values + derivatives). Direct access to fields (.dx etc.) can be replaced by free functions found by ADL.

A general, long-term solution would be to implement the concept of a jet, although ideally, that would require some fixed-size multilinear algebra utilities.