No help message when mandatory argument is missing.

[bhigy]

PlataleaConfig deactivates the help message by default and has a method called enable_help() to reactivate it. The way it is implemented now, arguments are parsed first and we check for the presence of the flag -h. However, if mandatory arguments aren't provided, parsing will fail and send an error message before we get a chance to trigger the help message.

This issue can be replicated by, e.g., running following command:

python -m platalea.utils.evaluate_net -h

[egpbos]

Related to #62

[egpbos]

As discussed during Teams meeting:

• We may not need the PlataleaConfig class at all anymore, but could just replace it with a regular configargparse parser. Let's try just replacing the whole thing.
• Otherwise, we don't need the enable_help anymore anyway, because it was used to be able to parse twice (which we did in the library code first and then second in the experiment), but we don't do that anymore.