Add function to set reservoir head patterns

[hnz92]

See title.

I couldnt find a way to set a reservoir head pattern and it took me some time to figure out that calling setNodeDemandPatternIndex on a reservoir ID will do just that.

d.addNodeReservoir("test_reservoir", (1064, 1203))

pidx = d.addPattern("test_pattern")

d.setNodeDemandPatternIndex(d.getNodeIndex("test_reservoir"), pidx)

d.setPattern(pidx, [0,1,2])