model and architecture bug

[federicazanca]

at the moment if model AND architecture are not provided anywhere the default is taken from janus. this is wrong, I just need to add a validation that at least one between arch or model should be given either as an input or in the config.
Then the thing is already written in a way that
if the architecture only is given , a default model is used (which is the same as janus indeed, but we handle it here in aiida level)
if the model only is given, the architecture should be already defined in the model. This actually is only true if the model is give through modeldata, if the model is given in the config, the architecture will be janus default, so probably we also want to change this.