grep used to find .tendl files

[shimwell]

Admittedly I've not tinkered with this repo for a while but while looking into the feasability of adding TENDL 2015/ 2017 and 2021 chain files I noticed this part of the code that looks up the transportable endf files. I think we can just grep for these files and don't need to have a hard coded list. Perhaps I am missing something here?

[paulromano]

@shimwell The reason that we have that separate JSON file of transport nuclides is explained in the description of #41. TENDL itself includes >2000 incident neutron files and so the predetermined list represents a reasonable subset of nuclides (630) to use for depletion purposes.

If we get to the point where the depletion chain can list all possible reactions but a user has a way to specify a subset of nuclides/reactions they want to track during depletion, that would be a good point to switch over to the approach you've proposed here.

[shimwell]

I guess all the nuclides and all the reactions is really going to blow up the memory. I guess we keep this PR in draft then just in case we get that filtering of depletion chain feature at a later date