cmake: namespace vtkm targets

VTK-m namespace for its exported CMake targets

VTK-m exported CMake targets are now prefixed with the vtkm:: namespace.

What it means for VTK-m users

VTK-m users will now need to prepend a vtkm:: prefix when they refer to a VTK-m CMake target in their projects as shown below:

add_executable(example example.cxx)
# Before:
target_link_libraries(example vtkm_cont vtkm_rendering)
# Now:
target_link_libraries(example vtkm::cont vtkm::rendering)

For compatibility purposes we still provide additional exported targets with the previous naming scheme, in the form of vtkm_TARGET, when VTK-m is found using:

# With any version less than 2.0
find_package(VTK-m 1.9)

add_executable(example example.cxx)
# This is still valid
target_link_libraries(example vtkm_cont vtkm_rendering)

Use with care since we might remove those targets in future releases.

What it means for VTK-m developers

While VTK-m exported targets are now prefixed with the vtkm:: prefix, internal target names are still in the form of vtkm_TARGET.

To perform this name transformation in VTK-m targets a new CMake function has been provided that decorates the canonical install routine. Use this functions instead of install when creating new VTK-m targets, further information can be found at the vtkm_install_targets function header at CMake/VTKmWrappers.cmake.

Extra

• I also went ahead and made the examples cmakelists.txt to link against the vtkm:: exported targets rather than the vtkm_ internal targets. The reason of this is that I believe new users will sometimes use this examples as a reference and using the internal targets will confuse them.
• Unrelated to this MR, I changed the CUDA CI build jobs to not to request a CI runner with cuda-rt. As soon !2943 (closed) is merged, the commit including those build changes will disappear when merging this MR.

fixes #625 (closed)