STYLE: use typedef rather than inheritance

itk::MetaDataDictionary::MetaDataDictionaryType was inheriting from std::map,
now we use directly a typedef.

DOC: fix function name error in Doxygen documentation for ExposeMetaData

Change-Id: Ie4ce42a99f13a9e42f407d4a2109c731d0b0b3d0
Merge topic 'MetaData'

03a8d53b STYLE: use typedef rather than inheritance