Implement `get_config`

This is got a large MR for implementing the get_config command. As it turned out, there was some refactoring for existing classes needed.

TODO:

• Add exceptions for unused keys
• Define behavior for unused keys
• Add getUsedKeys() and return that for get_config
• Add unit tests

@simonspa @hanno.perrey an open question is how we handle unused keys in the configuration. The problem is that at that point, the user initialization function (or reconfigure function) already passed successfully. I see three options:

1. Simply log a warning

• pro: simple typo can be fixed quickly
• con: can be easily missed/ignored

2. Go to ERROR state

• pro: it is obvious that there is a problem
• con: ERROR state is undefined and requires manual intervention

3. Go to SAFE state

• pro: in a defined state, requires new configuration to exit it
• con: requires a new transition from initializing and reconfiguring to SAFE

I'm leaning towards option 3: it is hard to miss/ignore and at the same easy to recover from