Signed-off-by: Felix Schindler <felix.schindler@wwu.de>

This can be used for instance in the context of a static_assert to check
if a given function is derived from LocalizableFunctionInterface. Since we
do not only check whether the function is tagged as LocalizableFunction but
in deed derived from LocalizableFunctionInterface we need the helper class.

Thus new functions do not have to implement this.

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... :)