deprecated header warning not very helpful

Currently deprecated headers include a #warning statement after the inclusion guards. This means that the warning is triggered only during the first inclusion. This might even be in one of the core modules, in case we have to support the deprecated interface (like in the case of function.hh).

I think it would be better to

• trigger this warning upon every inclusion f the header, this helps keeping track of all places which need to be updated
• add an #ifndef DONT_WARN_DEPRECATED_HEADER (or similar) to explicitly disable the warning for a particular #include statement and use this t avoid warning in those spots of the core modules, where we still have to use the header.

I can prepare an example patch for dune-common...