Separating sitkImage.cxx for compilation performance

Keep the non-templated code in sitkImage.cxx, separate the template
functions into sitkImage.hxx, and explicitly instantiate them in
sitkImageExplicit.cxx.

Change-Id: Iff9fa085062832c03f9b157e034e6fd97bd07e9d
Adding access methods to image's meta-data dictionary

Change-Id: Ic7d85f125ad84907526e37645440231e97d08e02
Merge topic 'SIMPLEITK-435_AddMetaDataDictionaryAccess' into next

1a3025f Adding access methods to image's meta-data dictionary
3cf4d8a Separating sitkImage.cxx for compilation performance