Create data exploration tutorial

[sfluegel05]

It would be useful for people new to chebai to have a short notebook that explains how the data is structured. This notebook should include:

• Instantiation of a data class (with explanation of common init parameters, e.g. chebi_version)
• prepare / setup methods (why are they needed?)
• a closer look at the different data files created
  • how is the file content structured? E.g. for a dict: what are the keys?
  • which information is stored in this files?
• for an example molecule: show different encodings (e.g. parsed SMILES, SELFIES)
• other things that might be useful