Refactor, clean up and document code

[sveinugu]

• https://app.zenhub.com/workspaces/elixir-oslo-5e2fa59480ef93e1d919fc6d/issues/fairtracks/fairtracks_augment/16 should provide possibilities to refactor/clean up code quite a bit.
• Replace simple constants with the strings themselves, as the constants are a bit superfluous. E.g. TERM_ID = 'term_id' and similar.
• Change getFromDict to use get()-semantics, returning None if the property is not present. Needs to change all usages, but will remove a large amount of try/except clauses.