Move class Interval into the namespace Dune::Solvers

This should avoid clashes with the copy in dune-fufem.

[[Imported from SVN: r12799]]

dune/solvers/common/interval.hh

 #ifndef DUNE_SOLVERS_COMMON_INTERVAL_HH
 #define DUNE_SOLVERS_COMMON_INTERVAL_HH
 
-// Note: This file exists both in dune-solvers and in dune-fufem. To
-// avoid redefinitions, please make sure that they have *identical*
-// include guards.
-
 #include <iostream>
 #include <algorithm>
 
@@ -12,7 +8,9 @@
 
 #include <dune/common/array.hh>
 
+namespace Dune {
 
+  namespace Solvers {
 
 /** \brief Encapsulates a closed interval
  * \tparam field_type The type used for real numbers
@@ -79,5 +77,9 @@ inline std::ostream& operator<< (std::ostream& s, const Interval<field_type>& in
     s << "[" << interval[0] << ", " << interval[1] << "]";
     return s;
 }
+
+  }   // namespace Solvers
+
+}   // namespace Dune
   
 #endif