A large portion of the VTK-m filters are now compiled into the
vtkm_filter library. These pre-built filters now don't include
the respective hxx file to remove the amount of template
instantiation users do.
To verify that this removal reduces compiler memory ( and maybe time)
I profiled the compiler while it building filter tests in debug mode.
Here is a selection of results:
```
CleanGrid 10.25s => 9.01s, 901MB => 795MB
ExternalFaces 13.40s => 5.96s, 1122MB => 744MB
ExtractStructured 4.69s => 4.75s, 492MB => 492MB
GradientExplicit 22.97s => 5.88s, 1296MB => 740MB
```
Generally, fields that have a WHOLE_MESH association might be valid even
if the structure of the mesh changes. Thus, it makes sense for filters
to pass this data pretty much all the time.
Also cleaned up some code and comments to make the relationship between
`MapFieldOntoOutput` and `DoMapField` more clear.
The version of `Filter::Execute` that takes a policy as an argument is now
deprecated. Filters are now able to specify their own fields and types,
which is often why you want to customize the policy for an execution. The
other reason is that you are compiling VTK-m into some other source that
uses a particular types of storage. However, there is now a mechanism in
the CMake configuration to allow you to provide a header that customizes
the "default" types used in filters. This is a much more convenient way to
compile filters for specific types.
One thing that filters were not able to do was to customize what cell sets
they allowed using. This allows filters to self-select what types of cell
sets they support (beyond simply just structured or unstructured). To
support this, the lists `SupportedCellSets`, `SupportedStructuredCellSets`,
and `SupportedUnstructuredCellSets` have been added to `Filter`. When you
apply a policy to a cell set, you now have to also provide the filter.
VTK-m now provides the following filters with the default policy
as part of the vtkm_filter library:
- CellAverage
- CleanGrid
- ClipWithField
- ClipWithImplicitFunction
- Contour
- ExternalFaces
- ExtractStructured
- PointAverage
- Threshold
- VectorMagnitude
By building these as a library we hope to provide faster compile
times for consumers of VTK-m when using common configurations.
