Rename isNumber to Solvers::IsNumber

The namespace should avoid collisions once IsNumber is accepted into
dune-common.

dune/solvers/common/typetraits.hh

@@ -4,13 +4,15 @@
 #include <type_traits>
 
 namespace Dune {
-template <typename T>
-struct isNumber
-    : public std::integral_constant<bool, std::is_arithmetic<T>::value> {};
+namespace Solvers {
+  template <typename T>
+  struct IsNumber
+      : public std::integral_constant<bool, std::is_arithmetic<T>::value> {};
 
-template <typename T>
-struct isNumber<std::complex<T>>
-    : public std::integral_constant<bool, isNumber<T>::value> {};
+  template <typename T>
+  struct IsNumber<std::complex<T>>
+      : public std::integral_constant<bool, IsNumber<T>::value> {};
+}
 }
 
 #endif