ENH: Checkpoint and CheckpointCalculator
This merge request includes a new ase.calcualtors.checkpoint module which provides two new classes: Checkpoint, for checkpointing individual values, and CheckpointCalculator, a wrapper that adds checkpointing to any ASE-compatible calculator. Checkpoints are stored using the ase.db database.
This code is being imported from matscipy, maintained by Lars Pastewka and myself. (https://github.com/libAtoms/matscipy), where is has been in use for around 1 year.
A test set and documentation strings including examples of usage are included.