Adding regular mesh based shut down dose rate example

[shimwell]

Not ready yet but this PR aims to add a mesh based R2S shut down dose rate simulation to the examples. There are a few more parts that need to be added to openmc before this PR can be finished. Currently the meshes that contain two or more materials are not modelled correctly

[shimwell]

Paul has a nice PR that solves the multiple materials on a single mesh voxel problem.

openmc-dev/openmc#2971

usage is
list_of_MIXED_materials_one_for_each_voxel = mesh_void.get_homogenized_materials(model, n_samples=1_000_000)

so it should be possible for me to finish of this example now

@rlbarker fyi

[shimwell]

The recent improvements in openmc reduce the complexity of this time of simulation considerably many thanks to @paulromano

I've tidied up the example and it is more concise but it is still not quite ready for use yet, sorry @rlbarker

Either I have made a mistake with my meshes or there is an openmc bug that when using a mesh source and a mesh tally then they both end up using the same mesh instead of different meshes. In this case I've got two meshes and want to make the source on a course regularmesh and tally the dose on a finer regular mesh but it appears to be using just the one mesh. I can look into this more.

[shimwell]

we will need the features in this PR as well openmc-dev/openmc#3000