ENH : implement SetFieldAsInt and GetFieldAsInt. Add a method copyFieldList. Implement the constructor by copy

      - Implement the methods SetFieldAsInt and GetFieldAsInt to handle
        correctly the fields with int type
      - Implement the constructor by copy since the operator = is implemented
      - Implement CopyFieldList method to be able to copy a field list from a kwl to
        another without copying all the MetaDataDictionary (take as example two vdata in
        different Coordinate systems)