This revamps Filter API to improve support for multiblock dataset and
distributed use-cases. Filters now operate on MultiBlock alone thus
`Filter::Execute(MultiBlock&)` is the primary method.
`Filter::Execute(DataSet*)` is only provided as an convenience that
internally puts the dataset into a MultiBlock and calls the primary
method.

Subclasses can provides PreExecute and PostExecute overrides to handle
init/reduction operations for filters with operating on multiple blocks.
Since blocks can be distributed, filters are expected to handle that as
well.

Subclasses can provide `PrepareForExecution(MultiBlock*..)` to customize
behavior for handling multiblock inputs. Default is to simply loop
over blocks.