Last checkpoint

Current situation

When training, various checkpoints are stored in the same subfolder checkpoints. Some have a timestamp and other suffixes (e.g. Testing_100k_2023-08-31_11-45_NSamples_1000_49_0.00.ckpt), and there is one with a name "last".
However, when running training multiple times, this last.ckpt is not overwritten, but new files are created named last-v1.ckpt, last-v2.ckpt etc. in the same folder.

Proposed change

Having only one last.ckpt would be useful because one doesn't have to change the path in the script if one is happy with the default. Optionally and additionally there could be a run-specific "last" checkpoint with the same timestamp and suffix formatting as the other intermittent checkpoints.