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
```