Build the heater/pot/sensor rig as one unit, picked by plant_name alone

heater_name/sensor_name/plant_name were three independent config keys,
each picked via its own factory - in practice sim and real hardware
are never actually mixed and matched, so this could (nonsensically)
disagree, e.g. a real heater paired with a simulated sensor.

Add PlantFactory (components/plant/plant_factory.py): plant_name alone
now builds the whole rig together. "sim" gets HeaterSim/Pot (the
modeled plant, as before)/TempSensorSim. Anything else gets
HeaterHendi/PotReal/TempSensor_max31865 - PotReal is a new, deliberately
unmodeled Pot for the real, physical kettle (components/plant/pot_real.py):
the real temperature comes straight from the real sensor, not from a
model, so it just accepts and ignores set_plant_params()/
set_ambient_temperature()/initial() and reports no temperature of its
own, satisfying PotTask/SudTask's interface with nothing to actually
simulate.

heater_name/sensor_name are gone from config.json.templ; server/brewpi.py
delegates to HeaterFactory/TempSensorFactory lazily from inside
PlantFactory, same as before, so real hardware's spidev/pyserial deps
still aren't needed just to import the module.

config.json.templ

@@ -4,8 +4,6 @@
 	"dt": 1,
 	"sim_warp_factor": 50.0,
 	"stirrer_name": "sim",
-	"heater_name": "sim",
-	"sensor_name": "sim",
 	"plant_name": "sim",
 	"pid_type": "Smith"
 	},