make constructor with a single integer argument explicit

[[Imported from SVN: r4712]]

common/bitfield.hh

@@ -22,7 +22,7 @@ namespace Dune {
     BitField() : std::vector<bool>() {}
 
     //! Constructor with a given length
-    BitField(int n) : std::vector<bool>(n) {}
+    explicit BitField(int n) : std::vector<bool>(n) {}
 
     //! Constructor which initializes the field
     BitField(int n, bool v) : std::vector<bool>(n) {