Fix forces acting on constrained atoms when reading from extxyz database

[yantar92]

I have noticed that forces from xyz training inputs (example: attached) are not always loaded correctly.
When the inputs contain structures with constrained atoms, the forces used for training are set to 0 instead of the real values obtained from the DFT data.

This seems to happen only with newer ASE versions, after ASE started storing the constraint info in the xyz data. See https://ase-lib.org/changelog.html#id18

In

mace/mace/data/utils.py

Line 276 in 667eee4

atoms.arrays["REF_forces"] = atoms.get_forces()

, get_forces method is used, which automatically sets forces to 0 by default, if constraints are present. See https://ase-lib.org/ase/atoms.html#ase.Atoms.get_forces

AtomsToGraphs might also be affected, but I did not check in details. There are also a couple more instances of get_forces in the code, but those look safe. (Although, explicitly passing apply_constraint=False might be a good measure if that's what is implied in the code).

Potentially related: #1148

CC: @anfeus

data.xyz.gz