these are "traits" classes and provide a way to access the value of
function evaluations, conditional access to its value (i.e., to
forward them if the target object of an assignment includes the
derivatives or use the function value if not) and some algebraic
functions.
the main idea is to be able to abstract the differences between plain
scalars and function evaluation...
they used to be in opm-core, but this allows to be more flexible with
the dependency order: What's now called "opm-core" can easily depend
on opm-material which might come in handy for the refactoring.
Besides moving in classes from opm-core, the infrastructural code
which was still in opm-material is moved to the directory
opm/material/common. The intention is to collect these classes at a
central location to make it easy to move them to a real "core" module.
(if this is ever going to happen.)
