Update mapping process + Cardano mapping info

[LadyChristina]

All Submissions:

• Have you followed the guidelines in our Contributing documentation?
• Have you verified that there aren't any other open Pull Requests for the same update/change?
• Does the Pull Request pass all tests?

Description

Updated Cardano cluster information, so that it uses (unique) pool hashes as keys.
Also updated Cardano identifier processing, so that it doesn't store duplicate tickers (which can lead to false attribution)
In the process, I also made some improvements to the mapping classes, so that the mapping method is reported more accurately (now the options are 'known_identifiers', 'known_addresses', 'known_clusters', 'known_legal_links', 'fallback_mapping') and updated the documentation and tests accordingly.

Note that the reason why I changed some values in the end_to_end test is because of the additional cluster info we now have on Cardano, which changes the distribution of blocks to entities (now there is one entity that produces two blocks in the sample file). In general, I believe the tests shouldn't be based on these files, but this is sth we can address in the future.

Resolves #135

Checklist

Update Mapping Support Information Submissions:

• For which ledger do you update the mapping information?
  • Cardano
• What mapping information do you update?
  • identifiers
  • addresses
  • clusters
  • legal links
• Did you update the tests (if needed)?

[dimkarakostas]

LGTM - fix the nit and merge