Fitness function modularity changes.

[JedS6391]

This PR builds upon the existing fitness function implementation, to provide a more modular API.

The main changes are:

• The fitness function type no longer operates on a program and a set of fitness cases. Instead, it takes a set of program outputs and a set of fitness cases.
• A fitness context builds a set of program outputs for the fitness cases and applies the fitness function appropriately.

In further work, it may be worthwhile to consider modularising this to allow for custom fitness contexts, for cases where multiple program outputs are used as there is no way to currently handle this.

Also, a way should be given to determine which register is used as the output register (probably best done through a field in the config).