Fix start values for some hot water inputs

The start value queries for condensing combi boilers and coal-fired
heaters referenced the "households_useful_demand_hot_water" node,
however the two supply nodes are no longer directly connected to the
useful demand node.

The queries have been updated to reflect that they are now connected to
"after solar heater and add-on" useful demand node.

Closes quintel/etmodel#1786