adding BoundingBox.extent and RegularMesh.bounding_box

[shimwell]

I'm plotting a few regular meshes at the moment and would find it useful to be able to easily get the extent of the regular mesh.

Adding a bounding_box property to the RegularMesh and adding the extent property to the BoundingBox would make it easier when plotting.

example usage.

plt.imshow(
    slice_data,   # contains 2d slice of 3d regular mesh tally data
    origin='lower',
    norm=LogNorm(),
    extent=mesh.bounding_box.extent['xy']   # <- this PR enables getting the extent like this
) 

[shimwell]

Sorry for this mess of a PR, I accidentally merged a few branches together. I've tidied it up now. Great example of why squashing commits on merge is the way to go 😄

[paulromano]

Simple enough -- thanks @shimwell!