ENH: env variable for execution space

[tylerjereddy]

Currently we have an example module: examples/ExaMiniMD/src/examinimd.py that does this:

        ...
        space: Optional[str] = os.environ.get("PK_EXA_SPACE")
        if space is not None:
            pk.set_default_space(pk.ExecutionSpace(space))

I think it would be helpful to have an "endorsed"/official way to use environment variables to swap between execution spaces instead of rolling custom code each time.

This may be particularly relevent to python runtests.py ..., because it is not particularly convenient to manually add shim code to swap the execution space if you want to test our current status on GPUs without most of the code running on the host.

If we can't agree on something for the main library, I'd at least want to see runtests.py capable of doing this.

Something similar for enabling/disabling UVM may also be useful, though perhaps not quite as fundamentally important as testing an entire execution space to start.

[JBludau]

Something similar for enabling/disabling UVM may also be useful, though perhaps not quite as fundamentally important as testing an entire execution space to start.

Sidenote: the classic enable uvm which makes CudaUVM aliasing CudaSpace is gone with Kokkos 4.0. Nevertheless, there is SharedSpace introduced which also works for Hip an SYCL ...

Maybe we should think about something like pk.setup with defaults or stuff being read to from the env when pykokkos is loaded. This way it can still be changed for individual scripts