Documentation revamp #40

We currently have very little documentation, and what we do have is largely oudated. This issue outlines the items that should be added.

• up to date README file
• inline documentation on all exported types
• inline documentation on all exported functions (i.e. InteratomicPotentials API functions)
• instructions in the Documenter docs explaining the type hierarchy and how to interact with existing implementations
• Instructions in the Documenter docs explaining how to create a new implementation of each type (empirical, trainable, non-trainable, abstract)
• examples of usage