Matgl

* Model serialization has been completely rewritten to make it easier to use models out of the box.
* Convenience method `matgl.load_model` is now the default way to load models.
* Added a TransformedTargetModel.
* Enable serialization of Potential.
* IMPORTANT: Pre-trained models have been reserialized. These models can only be used with v0.5.0+!

* Pre-trained M3GNet universal potential
* Pytorch lightning training utility.

* Major refactoring of MEGNet and M3GNet models and organization of internal implementations. Only key API are exposed
via matgl.models or matgl.layers to hide internal implementations (which may change).
* Pre-trained models ported over to new implementation.
* Model download now implemented.

* Fixes for pre-trained model download.
* Speed up M3GNet 3-body computations.

* Pre-trained MEGNet models for formation energies and band gaps are now available.
* MEGNet model implemented with `predict_structure` convenience method.
* Example notebook demonstrating pre-trained model usage is available.

* Initial working version with m3gnet and megnet.