cmake: split vtkm_filter into common|extra|contour|gradient

closes: #504 (closed)

There is a limitation in Windows builds using VS2019 where libraries cannot be bigger than 4GiB. This is normally not an issue but in VTKm due to its strong template usage libraries can reach that size.

The VTKm filter library is can easily reach that size and it will halt the build

This MR tries to avoid reaching those sizes for now by splitting the filter library into four smaller libraries.

The proposal scheme is:

It splits vtkm-filter into ->
  - vtkm-common  (Classes that are dependencies of other filter libs)
  - vtkm-contour (Contour class and its instantiations)
  - vtkm-gradient (Gradient class and its instantiations)
  - vtkm-extra   (Classes other than Contour or Gradient that are not dependencies)

Windows filter libraries sizes:

Libraries	Current	this MR
vtkm-filters	1.3 GB	-
vtkm-common	-	308 MB
vtkm-extra	-	424 MB
vtkm-contour	-	639 MB
vtkm-gradient	-	422 MB

Along this new filter sublibraries new exports headers are now generated as seen here:

→ find . -iname '*export.h'                                                                                                                                                                                         
./include/vtkm/cont/vtkm_cont_export.h
./include/vtkm/source/vtkm_source_export.h
./include/vtkm/worklet/vtkm_worklet_export.h
./include/vtkm/filter/vtkm_filter_extra_export.h
./include/vtkm/filter/vtkm_filter_common_export.h
./include/vtkm/filter/vtkm_filter_gradient_export.h
./include/vtkm/filter/vtkm_filter_contour_export.h
./include/vtkm/rendering/vtkm_rendering_export.h
./include/vtkm/io/vtkm_io_export.h