Warn when there are leftover extraneous files in input/output folder

[wil93]

It would be useful to show a warning when there are leftover extraneous files in input/output folder (or maybe even anywhere, if we detect that a file isn't "used" and is just there being ignored it's probably good to warn)

My use case is:

1. Iterate editing gen/GEN multiple times until you're happy with it
2. It can happen that at some point the file had, for example, 20 testcases, but when you finalized it it only had 15
3. The files input/input0.txt through input/input14.txt are actually up to date and necessary, while the remaining 5 files are just sitting there, not being deleted by TMR

It would be nice if at that point TMR would say "hey you probably don't need these files: input/input15.txt, input/input16.txt, ..., input/input19.txt"

Or would it make sense if TMR just deleted the input/output folders before starting to generate inputs?