Add energy measurements support

[cappadokes]

This PR introduces a new, --track-energy option for reporting energy consumption measurements instead of execution time, memory etc.

ATTENTION: --track-energy needs changes in pyperf in order to work, since pyperf is the real place where benchmarks are run.

[cappadokes]

This PR's title doesn't match what the code does (it seems to be adding a --track-energy flag).

To be precise, the code both fixes the --inherit-environ issue and it also adds a --track-energy flag. I apologize for not splitting this in 2 separate PRs, but the changes under discussion are partly interdependent (--track-energy needs --inherit-environ to work, though the converse does not hold).

I will soon follow up with a documentation commit for clarification. I will also change the PR's name so as to contain both points.

[ericsnowcurrently]

@cappadokes, please put the --track-energy addition in a separate PR. We will merge the fix for --inherit-environ first.

[cappadokes]

@cappadokes, please put the --track-energy addition in a separate PR. We will merge the fix for --inherit-environ first.

@ericsnowcurrently done. --inherit-environ is here now.