Added simpler Gather(vtkDataObject*) method.

vtkCommunicator::GatherV() required the destination process to know what
data types to expect. This was unnecessary. There's enough information
in the marshalled data object received from  the sender side to deduce
that. Adding new vtkCommunicator::Gather() and
vtkMultiProcessController::Gather() methods that provide a simpler
interface.

Updating GenericCommunicator test to test for the new API.