`-s` command line option is not range checked

[loganharbour]

Description

Passing the -s/--threads command line option is not range checked. If a value is not provided, we get a logic error thrown instead of a useful error message.

In particular, see

openmc/src/initialize.cpp

Line 263 in 58f2a21

int n_threads = std::stoi(argv[i]);

argv is accessed out of range when executing something like openmc -s.

Alternatives

Add a range check when accessing command line arguments to throw a better error.

Compatibility

Improves user experience.

[pshriwise]

Thanks for noting this @loganharbour. I'll add that the executable will only recognize flags if they come before positional arguments as well.

There's a whole overhaul of our command line arg parsing that I'd love to take on sometime. Perhaps using a tool like https://github.com/p-ranav/argparse/tree/master, which I've used in the past in other projects.