The connected components filters had some simple issues that were
inconsistent with the operation of other filters.
* Set the output to the OutputFieldName defined in the filter rather
than hardcoding it to "component". The default value is still
"component", but now you can change it.
* Make sure the output field association is correct. The field
association for CellSetConnectivity is always cell and the field
association for ImageConnectivity is always point.
* Check that the field association for the input field of
ImageConnectivity is point. The filter will fail if it is not.
* Make both filters inherit from FilterCell instead of FilterField.
The superclasses are similar but the latter allows the user to set the
active cell set correctly.
* Properly get the cell set specified by GetActiveCellSetIndex.
Previously it was set to the active coordinates, which is wrong.
