- for this purpose, add new constructors and build_gradients method taking
a FieldMatrix and get "gradient" key from config as FieldMatrix

This reverts commit e52259a4b3c6f7b6ee6b3e97af9ffdc32eff20bb.

Paragraph 12.8/15 of the C++11 Standard:

The implicitly-defined copy/move constructor for a non-union class X
performs a memberwise copy/move of its bases and members

There is finally only one FunctionProvider, all specializations are gone.
The price for this is one additional line in all functions that should be
provided which is acceptable. See the added documentation of
FunctionProvider on how to proceed for new functions. The only drawback is
that there are still four places in functions.hh that need to be touched.
This is left as an exercise for someone who fancies static loops over tuple
types (not a tuple object), I am too tired now... :)

refs #282

refs #282

this requires fixing by @ftalbrecht

- mark variable as unused to hide warning
- change Dune::IOError to Exceptions::configuration_error
- get order as size_t instead of int and change its default value

-rename namespace Function to Functions
-rename class Functions to FunctionsProvider