API to get the absolute start time of a single phase

[alaurenzi]

Even if this is outside the horizon.

Expected usage (by me)

phases = timeline.getPhases()  # gets all defined phases (even after horizon ends)
last_phase = phases[-1]
k0 = last_phase.getStartNode()  # the absolute start node of this phase, where 0 is the current horizon start
N = last_phase.getNNodes()  # the duration of this phase, in nodes

This way, the first node for which no phase is defined shall be k0 + N. This is useful per me