Fix: Inline free function operator>> in NumProc.

A non-templated, non-inlined free function may generate
a non-weak symbol which causes linker errors due to
duplicate symbols when the file is used in more than one
compilation unit.

Inlining (which is implicit to non-fully specialized
templated functions) generates weak linker symbols and
hence allows for usage in several compilation units
without linking errors.

dune/solvers/common/numproc.hh

@@ -30,6 +30,7 @@ class SolverError : public Dune::Exception {};
 
     };
 
+    inline
     std::istream& operator>>(std::istream &lhs, NumProc::VerbosityMode &e)
     {
         std::string s;