Parallelism assumes direct `west.h5` file access

[jdrusso]

Currently, some parallelized sections of msm_we still assume they'll have direct access to the west.h5 files being analyzed.

For example, taking the discretization: in do_stratified_ray_discretization, I pass an iteration to the remote worker, and the remote worker is responsible for retrieving the data.

This was an intentional decision, to avoid having to send lots of data. However, this means we assume direct access to the west.h5 files... This prevents you from using a remote cluster for Ray work.

To me, it seems like the options are:

• Copy all the west.h5 files to the remote worker, maybe in the restart plugin? Ray supports this in init via runtime_env argument, but it has pretty conservative limits on file size. This has the drawback of requiring a large file copy.
• Send iteration data directly, rather than having each worker read it. This, on the other hand, has the drawback of requiring lots of data to be sent over the network.

[jdrusso]

This is partially addressed in #39 which provides an explicit base path to west.h5 files. As long as the same path is available on the remote workers, everything is fine.

This is in contrast to assuming the relative path is available, which depends entirely on the working directory of your Ray workers (and probably isn't generally true)