scaling of loss with number of atoms (and other general cleanup?)

[bernstei]

We should figure out how to scale the loss with number of atoms. For GAP and ACE, we've settled on

loss = (w / sqrt(N)) ^2 (E-Eref)^2

for both energy and virial (extensive). From what I can tell, MACE uses `

loss = wM ((E-Eref)/N)^2

In addition to a (trivial) square on the magnitude of the weight, these do not scale the same with number of atoms. They can be mapped to each other through

wM = N w^2 

The former assumes that the energy errors are uncorrelated. The latter assumes something else, but I'm not sure how to define it in terms of correlations. But, for example, if you double the system size you get twice as many forces, but the same scale E/N, so same contribution from the energy.

Maybe the scaling should be selectable by the user?

Also, in general, there are many possibly duplicative loss functions coded up, and it would be very nice if they were unified. I can even imagine all of them being implemented as huber, and recover the quadratic ones by setting the threshold to infinity.