Add utility for measuring power accumulators on guybrush

BUG=b:184976380
TEST=Running the script and collecting power measurements. These were
compared against bench power supplies and found to be accurate.

Change-Id: I3372f9f09496e0d735f8e1b97317e93782cc9290
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2874430
Reviewed-by: Rob Barnes <robbarnes@google.com>
Tested-by: Michael Ellis <michaelellis@google.com>
Commit-Queue: Michael Ellis <michaelellis@google.com>
8 files changed