Original DeclareInterpolation,UpdateInterpolation,DeclareState, ....
were all using a std::function as argument type for creation/updating
operation passed to those functions.

G.L. discovers during xField templatization that old compiler where in
default with the new signature of these arguments.
In fact the problem comes from the fact that in general we provides a
instance (xValueCreator, xStateDofCreator, ....) for these arguments.
Says we rely on the compiler to do some binding of the instance to the
std::function. With linked template instance things where to complex apparently
for g++ 4.8.2.

A more general approach is to consider a new template parameter for those
arguments. This way either object function, std:function, lambdas, ...
can be passed to these function. Now it's clear for the compiler and for
the user: what is passed is something that correspond to a function
with specific argument and returned value.
For DeclareInterpolation, ....
it's xValue<typename FIELD::value_t>* (const xValKey&) const

Those argument are passed by reference to insure that "creator" are
not copied. And to be able to call DeclareInterpolation,... with
on the fly creation of "creator" those arguments are considered as
const. This imply now that () operator of "creator" must be const.

This is why in this commit many other files have been modified to
respect this constness. Note that in most case "creator" () operator
were intrinsically const. For some, mutable data member were needed.
In all case considering that  DeclareInterpolation,... do not
change the "creator" is rather logical.