FEATURE: Bridge stratification_keys to observables_keys if needed

[RasmusSkytte]

Presently, it is required that keys are present in both stratification_keys and observables_keys to perform $key_join_filter().

However, there will be cases where keys that are not present in both can be mapped through a "bridge". diseasystore has begun the framework to implement this bridge, but it is not fully implemented.

This feature request requests that this bridging functionality be added to the code base.