Update to ASE interface and SchNetPack ASE calculator

[mgastegger]

Updated the SpkCalculator and ASEInterface to follow the input conventions of the MD calculators. Changes to both include:

• Instead of an instance of an AtomsConverter a neighbor list is passed. This allows better consistency when handing device and precision options
• Instead of passing units for all properties (energy, forces and stress) only units for energy and positions are required, the rest is derived automatically
• Updated shapes of PBCs and cell in the AtomsConverter
• Stress computation is now handled properly by the SpkCalculator

Moved the utility function _activate_stress from the SchNetPack MD calculator to md/utils/activate_model_stress and updated to account for changes in overall model structure and derivative handling.

Updated tutorials 2 and 3 to reflect the changes in the ASE interface.

[ktschuett]

i called this "_key" instead of "_label" in other modules. should be consistent one or they other

[NiklasGebauer]

I also use "_key" in the gschnet code, I think it is more precise than "_label"

[mgastegger]

Changed energy_label, force_label and stress_label to energy_key, force_key and stress_key, as well as position_units and energy_units to position_unit and energy_unit. Now everything is consistent with the atomistic module. The changes affect the ASE and MD calculators, as well as the MD configs.