Version 0.3.1

• add gpu training for NN model; set the gpu parameter of a calculator (e.g. CalculatorTorch(model, gpu=True)) to use it
• add pyproject.toml, requirements.txt, dependabot.yml to config repo
• change to use the furo theme for docs
• change to compute grad of energy wrt desc in batch mode (NN calculator)
• fix to set fingerprints_filename and load descriptor state dict when reusing fingerprints (NN calculator)