add model settings shortcut for training

[alinelena]

at the moment when you run mace_run_train you need to specify the architecture details.

would be nice to have --small --medium --large that automatically set all the arch details to defaults uses in mace_mp.

[gabor1]

This is a difficult choice. if we do that, then people will never experiment with the options, and when we decide to change them one day, everyone will complain about the incompatibility. We don't really know enough yet about the accuracy tradeoffs between channel number and the other hyper parameters (ell, L, body order etc)