Use argparse for reading arguments of hp_optimization/grid_search

[luator]

Use argparse instead of read_params_from_cmdline in the hp_optimization and grid_search scripts. This would give the following advantages:

• Auto-generated help for --help/-h.
• Implementation of read_params_from_cmdline could be simplified as it wouldn't need to handle both hp_optimization/grid_search and the job scripts anymore. (*)
• Code will be easier to maintain.
• Easier to extend for future features, if needed.

(*) In addition, we could consider to also use argparse inside read_params_from_cmdline to improve readability of the code.

[luator]

I think we should tackle this one as a prerequisite for #81. I assume it will not easily be possible to auto-detect whether a config file is meant for smart_settings or omegaconf, so we will need a flag to indicate it (something like grid_search --smart-settings config.json). This would be much easier if switching to argparse first.