BUG: Fix compile error related to "WrapValue" unit methods

Following commit r22055 [1], three new methods have been introduced:
- WrapValueWithPrefix
- WrapValueWithSuffix
- WrapValueWithPrefixAndSuffix

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22055

This was causing a build error on linux:

 In member function ‘std::string vtkMRMLUnitNode::WrapValueWithPrefixAndSuffix(std::string&)’:
 ../vtkMRMLUnitNode.cxx:183: error: no matching function for call to ‘vtkMRMLUnitNode::WrapValueWithPrefix(std::string)’
 ../vtkMRMLUnitNode.cxx:159: note: candidates are: std::string vtkMRMLUnitNode::WrapValueWithPrefix(std::string&)

Making the method const correct fixed the problem.

More details about the error are reported here: http://stackoverflow.com/questions/1565600/how-come-a-non-const-reference-cannot-bind-to-a-temporary-object

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22060 3bd1e089-480b-0410-8dfb-8563597acbee