Closes #1295 EventDetector for beta angle threshold crossing

Implementation of a BetaAngleDetector, triggering events when a beta angle threshold is crossed. The detector (and the corresponding FieldBetaAngleDetector) contain static methods which can be used to compute the beta angle directly, if needed.

Resolves issue #1295 (closed)

Note to reviewers: I thought about adding a BetaAngleUtility to the utils package to hold the computation of the beta angle. However the actual computation was small enough it didn't seem worth it. If it makes more sense to organize this code differently, please let me know.