Consumed energy output: instantaneous power might be cleaner

[mpoquet]

In exported file _consumed_energy.csv, the written instantaneous power on a line corresponds to the energy difference with the previous line divided by the time difference with the previous line.

This leads to power values of NA for the first power value, or when two lines are at the exact same time.

Enhancement fix: the current consumed power could be retrieved from SimGrid to match the simulation current consumption. This way a line meaning would be present focused and future oriented, instead of past-focused and past-oriented. This way, no NA value would exist.