Refactor Head

Refactored Head : removed unused methods, removed useless or redondant members, added methods for code reuse.
Some methods used side effects to share data through members, since they'va been removed, this is no longer the case.
It's easier to read and understand Head's behavior.
Fixed a bug where a newly created context was added to the list of activated neighbors, resulting in a division by zero in getAverageRegressionPerformanceIndicator.