Plotting utility updates

[AntObi]

• PCA and t-SNE plots are now automatically coloured by the element's metallic classification
• Replaced random_200.json with 'random_200.csv` which should contain 118 elements
• Atomic_Embeddings.from_json can now accept json files
• A new property Atomic_Embeddings.element_groups_dict has been added which contains information about the metallic classification of the elements
• examples/usage.ipynb and publications/metrics_plots.ipynb have been updated to reflect the ability to colour the elements.