Allow ProcessController to exist without a setpoint

[climategadgets]

Existing Behavior

ProcessController#setpoint must exist no matter what.

Corner Cases Not Covered

This behavior forces the system to assign a default setpoint at startup, which may or may not be reasonable - rather not; case in point, a "middle" value of 23C will be way too hot for heating in Europe, and way too cold for cooling in Arizona.

Desired Behavior

Missing setpoint value is translated into an error signal of type "value missing" on the output, and into null values on getSetpoint() and getError().

Related Issues

#267