Switch from GraphRegressor to DirectForcefieldRegressor

[benrhodes26]

A few updates:

• Remove GraphRegressor. We now use ConservativeForcefieldRegressor and DirectForcefieldRegressor.
• Add stubs for the new models. We still need to add the actual weights paths.
• Return a SystemConfig in addition to the model. We need this because some models use max_num_neighbors=20, whilst others use max_num_neighbors=120
• Make sure we use the SystemConfig everywhere, avoiding hardcoding.

• Make choice of base model in finetune.py configurable and update readme accordingly.
• Update Readme, fixing the example code snippets
• Update Readme to explain precision (float32-high vs float32-highest and choice of EdgeCreationMethod)

[DeNeutoy]

Generally LGTM, one idea about avoiding passing around extra config state

[benrhodes26]

LGTM