Enable the Ace loader to return a conformation index

[raimis]

Enable the Ace loader to return a conformation index

ds = Ace('.', paths='dataset.h5')
for s in ds.sample_iter(mol_ids=True):
    print(s)

Data(y=[1, 1], pos=[12, 3], z=[12], neg_dy=[12, 3], q=0, pq=[12], dp=[3])
...

for s in ds.sample_iter(mol_ids=True):
    print(s)

Data(y=[1, 1], pos=[12, 3], z=[12], neg_dy=[12, 3], q=0, pq=[12], dp=[3], mol_id='C4H5N3', i_conf=10355)
...

[RaulPPelaez]

LGTM.
There is no test coverage for this Dataset, right?

[RaulPPelaez]

For consistency you could change these lines:
https://github.com/torchmd/torchmd-net/pull/174/files#diff-56de3e28dd2b438171f4af7de59cc66305304d98e21b24faf4d744698d8c1aa3R269-R279
To use the i_conf property