Refactor scripts to tested package

[shimwell]

Just making this PR to see if people want to move this collection of scripts in the direction of a Python package, no worries if this is not wanted but I am keen to offer.

This has a few improvements over the current collection, addressing issues #5 #65 #69

• more testing
• more reuse of duplicate code
• easier for users to install
• offers new category of scripts (download only, no processing)
• improved documentation

[paulromano]

Thanks for proposing these changes, @shimwell. I'm still of the mind that putting all this into a proper package is overkill. In practice, the only time we (OpenMC dev team) run these scripts is when there's either a new release of a data library or a change in our HDF5 format that requires we regenerate things. They have never really been intended as "user" scripts; rather, we put them here as an example to users in case they have their own specific needs for processing (i.e., they could take one of these scripts and modify for their own needs). Users who just want the results of running these scripts should download our pregenerated libraries available on openmc.org. I'm open to some of the changes within (adding some simple testing, e.g.) but don't feel that the added complexity in this PR is really worth it for what's intended to be a simple set of scripts that we run on occasion (the KISS principle comes to mind).

[shimwell]

No worries, I wanted to offer the fork back to the original repo just in case. Totally understand that you want to keep things simple on this repo. I shall keep you posted if I think it is ever worth another look and if you see anything in particular over on the fork then ping me and I'll get it separated out and we can copy it over.