properly deal with network/optim (end of the ModuleGroup cleanup)

properly deal with network/optim (end of the ModuleGroup cleanup)

The network/optim was abusing the model description mechanism, because
it was introduced before the options could be restricted to a list of
values. That's a pretty old cruft :)

Moreover, the config mechanism (probably) allowed network/optim:TI but then
ignored it silently. Not nice for the users.