the reason there hasn't been any API until now is because of how inconvenient it was to expose a LSP-compliant API _and_ use the same base class for handling all the connection logic. This commit fixes that problem by abstracting shape handling away from BaseRail entirely, so that now it deals exclusively with connections. Deciding the shape of rail to use is now the job of the subclasses.
detector rail has fundamentally different functionality than activator and powered rails, so it's misleading to present the same APIs for both.
detector rail's 'powered' state is better referred to as 'activated', since it means the detector rail is actually _producing_ power, and not _receiving_ power.
