Conformance Rules class hierarchy to support multiple input and output columns

## Background
With the introduction of multiple input and output fields to the Mapping Rule, would be good to address this generally.

## Feature
Refactoring of the Conformance Rules class hierarchy to support multiple input and output columns. Thsi way it would be a general principle for the rules, where it applies.
**NB!** This will require data migration.

## Proposed Solution

* General trait for rules
* Trait for exactly one input field
* Trait for exactly two input fields
* Trait for multiple (variable) input fields
* Trait for no output field
* Trait for one output field
* Trait for one multiple field

_Open question:_ How to address a possible required mapping between multiple input and output fields?