Add ball detector to the intake subsystem

[jkleiber]

Idea: set up an array of sensors so we can determine if there are any balls left in the indexer.

Sensors: Those $1 ones we have. They are digital inputs.

Will update this issue once the scope is more defined, but for now assume we have multiple sensors and if any of them are triggered we have at least one ball. Else, we have 0 balls. This will be important for auto shooting potentially.