v0.2.0 Last version with depracated FLAX checkpointing

This version is the last mlff version that can deal with the depracated flax checkpointing system. Explanation see below.

With v0.7.0, flax deparactes their internal checkpointing system and moves to orbax. This has the consequence, that old parameter checkpoints can not be restored with the newest flax version anymore and, unfortunately, breaks backward compability. Starting from v0.3.0 mlff moves to orbax, thus older flax checkpoint are not restorable anymore.