`simulator` should be set in 1 place (better not in the validator)

[cattabiani]

the simulator is set in at least 2 places:

neurodamus/neurodamus/core/configuration.py

Line 265 in 6000470

if cls.cli_options.simulator:

neurodamus/neurodamus/core/configuration.py

Line 665 in 6000470

def _set_simulator(config: _SimConfig):

it is very confusing. It is also a break of paradigm: a validator should not set stuff. Also, it is strange that we set it in the class, not in an instance.

In addition it is very restrictive that the CLIOptions object has the enums that are mandatory. Why can't we have optional enums? If not there, we use None, otherwise the enum