MeterGroup.total_energy() should no longer use `full_results` unless
explicitly asked to.  This *should* fix issue #137 and the assertion
error in #133.

But we now have two new problems!

1) The last test I wrote (which I knew wasn't a perfect replication of
issue #137) turns out to be a separate issue which needs fixing.

2) We don't have a good test for issue #137.