Upgrade compiler on titan
We should investigate to upgrade the compiler on titan. gcc 4.8 is quite old.
This leads to some limitations during template deduction. The code is thus less compact as we have to help the compiler to figure out what is going on (see
DeclareInterpolation<xField<valtype> > and
DeclareState<xField<valtype> > where the template parameter could be removed with more recent versions of gcc).