Use default required-argument for subparsers

[wouterjdb]

Currently we have some custom code for handling the situation where the user has not supplied any arguments.

https://github.com/equinor/flownet/blob/c23c9adfc09374aa220cbec11fc339504c494bf1/src/flownet/_command_line.py#L208-L210

For python > 3.6 there is the option of setting required = True as an argument for the subparser. We should update this when we drop Python 3.6 support.