STYLE: Remove duplicated TestingExtractSliceImageFilter doxygen.

Change-Id: I3f2b6c296032bb2b21da275e5bc85b5003c600a8
DOC: Improved comments and reorganized code for IsolatedConnected

Added documentation to the AddSeed1() and AddSeed2() methods.
Updated the deprecated SetSeed1() and SetSeed2() method documentation
to point to these instead of to AddSeed(), which does not exist.  Also
reorganized the code to emphasize these Add methods.
Wrapped the deprecated SetSeed1(), SetSeed2(), SetUpperValueLimit(),
and GetUpperValueLimit() methods in
ITK_FUTURE_LEGACY_REMOVE preprocessor directives.
Updated the IsolatedConnected test to wrap calls to SetSeed1(),
SetSeed2(), SetUpperValueLimit(), and GetUpperValueLimit() with
ITK_FUTURE_LEGACY_REMOVE preprocessor directives.
IsolatedConnected example also updated to avoid use of deprecated
methods.

Change-Id: Icf934b3eb9273d4b334a48b268784c32f1c6d806
Merge topic 'IsolatedConnectedSeedDocumentation'

e142b842 DOC: Improved comments and reorganized code for IsolatedConnected
Merge topic 'testing-extract-doxygen'

5144b613 STYLE: Remove duplicated TestingExtractSliceImageFilter doxygen.