execution backend selection improvements

[cyrush]

Three Goals:

1. Expand and clarify options for selecting VTK-m and other execution strategies.
2. Improve about() and info() to make sure they show
3. In addition to forcing a specific strategy, Add a way to specify a precedence list

Naming of "backend" or execution strategies

At a high level, we have three cases:

device
openmp
serial

If you expand this out to include the underlying programing model, we expect to have:

cuda
hip
sycl
openmp
serial

If we enumerate the VTK-m specific options:

vtkm_cuda
vtkm_kokkos_hip
vtkm_kokkos_cuda
vtkm_kokkos_sycl?
vtkm_openmp
vtkm_serial

(vtkm_cuda and kokkos_cuda are different implementations, and users may have a strong reason to choose one or the other)

I propose we support an option that accepts a union of these cases:

device
cuda
hip
sycl
vtkm_cuda
vtkm_kokkos_hip
vtkm_kokkos_cuda
vtkm_kokkos_sycl?
vtkm_openmp
vtkm_serial
openmp
serial

Where device gives you general option that maps to either cuda | hip | sycl with some precedence in the rare case more than one exists.

Additionally, we should support an input list that selects what should be tried and the precedence:

Examples:

execution: ["device","openmp","serial"]

Would try to use a device runtime (cuda or hip or sycl) first, then openmp, then serial

execution: ["cuda","openmp","serial"]

Would try to use cuda first, then openmp, then serial

execution: ["cuda","openmp"]

Would try to use cuda first, then openmp and would not try any others

execution: ["device"] or execution: "device"

Would only try to use a device runtime (cuda or hip or sycl) and no others.