PLANNER-99: Planning entity tabu support for relative size

This code is a proof of concept and is up for review. It makes the following assumptions:

- Number of planning entities doesn't change throughout the solver phase.
- Current set of all planning entities can be accessed through the solver phase scope.

The selected approach is fatally flawed should these assumptions not hold. Additional considerations:

- The implementation creates two separate inheritance branches: absolute size tabus and relative size tabus. Are absolute size tabus useful now that we have relative size?
- There is no test case. The current implementation of planning entity relative tabu is not testable using the same mechanism used for planning entity absolute tabu. This is because the relative implementation uses score director, of which there is none in the test case.

It is starting to look like this feature is a must-have for MISTA, we need to make it work.