On all of these existence checkers, is thread safety going to be an issue? I can imagine this happening if there need to be concurrent reads/write/creates/deletes to temp files, hdf5 caches, etc. One issues I've come across in systems like this before is the reliance on `exists` when there can be race conditions around the file creation. I have no idea if it's relevant to this specific section of code -- and Blaze team may have thought of that already -- but figured I would note it too.