Fix solenoid ID assignment so it is consistent, correct, and clearly commented

We use retract and extend consistently in the ID assignments to describe how the pistons move. But the numbering should be consistent.

Here's the outlyer.

RETRACT_ID = 6
EXTEND_ID = 7

That has them reversed. If the function of the pistons is reversed, we should do it the mechanism control with a concise comment. As far as electrical is concerned these are instructions about how to connect the pneumatic hoses. If retracting a cylinder extends a mechanism, we can do the change in our software abstraction.