Add a PrinterHeaters class that can stores references to available
temperature sensors and stores references to instantiated heaters.
Add a extras/heater_bed.py file and delay instantiation of the
heater_bed object. This allows the heater.py module to be imported
earlier during the setup phase, and allows the PrinterHeaters class to
be available for registering sensors and heaters.
Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Kevin O'Connor