Add {{fpaPosition}} key.

With the motor position persistence in place, we can declare whether the FPA is at the in-focus position. We could add a "positionName" field to the ccdMotorN keys, but then there would be three of them.

Instead, I propose using the value from the named motor move command, currently one of: "focus", "center", "nearHome", "nearLimit", "unknown". In practice all we will care about is "focus" vs. anything else, but this could prove useful.