Allow problemtools to run recursively

[Matistjati]

Recursively check the input directory, and run on all discovered problems. A directory is a problem if it contains a problem.yaml file. A directory is never a problem if it is a subdirectory of a problem.

Should at the very least be present as a command-line option -r, but honestly, why not the default? Thoughts @Tagl ?

The usecase is that I want to run verifyproblem on all problems in a contest as a last sanity check before sending them off for installation.

[Tagl]

I just have a script that finds all problem.yaml files and does this. I run some extra checks and extract some info so I like to separately run for each problem. I think a command line option makes sense just to be explicit, I think the common case is still run it on a single problem.