Previously, all the ApplyPolicy functions had the same name and used
template resolution to figure out which one to use. This was pretty
clear at first when there was just one for fields and one for cell sets.
But then it grew to several different types, particularly for fields. It
was hard to look at the code and figure out which form of ApplyPolicy
was being used, and compilers were starting to get confused.
Resolve the problem by giving all the methods unique names to make it
clear which one you expect to be called.
By removing the ability to have multiple CellSets in a DataSet
we can simplify the following things:
- Cell Fields now don't require a CellSet name when being constructed
- Filters don't need to manage what the active cellset is
To help provide a better time writing VTK-m filter this streamlines
the CreateResult API to provide a focused set of methods, based on
how CreateResult has been used by existing filters.
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.
