Use Implicit arrays in Connectivity and CellValidator

Optimize memory footprint for vtkConnectivityFilter and vtkCellValidator by using some vtkImplicitArray for their output.

Concerning doxygen update

This is some markdown in doxygen, https://www.doxygen.nl/manual/markdown.html

Usually, line endings have special meaning in markdown. Avoids line break inside image declaration.

before:

updated: