Input pipeline fully out of core

[M-R-Schaefer]

While the recent input pipeline rework has already greatly reduced memory consumption, we can defuinitely decrease it further.
The current major RAM hog is the dict of ASE atoms, not the data pipeline. We can avoid this by reading the file on the fly in the generator instead of all at once, thereby avoiding the need to keep all atoms in memory.

[M-R-Schaefer]

Although this could certainly be done, the current PBP dataset allows for training of even the largest available datasets with 64 GB of RAM, which can be expected from a modern workstation.