Fix round-robin subpath sorting
Fix round-robin subpath sorting

Subpaths were being "round-robin"ed using the *head* rather than the
left nodes. This means that the calculator was alternating between
electricity, gas, and heat technologies rather than selecting one
technology from each endpoint in turn.

This resulted in identical endpoints with different loads since one one
node an electricity technology would be calculated first, and on the
next node a gas technology would be first.

Closes #1153

quintel/etmoses,quintel/etmoses,quintel/etmoses,quintel/etmoses,quintel/etmoses