MeterGroup.load() now averages PHYSICAL_QUANTITIES_TO_AVERAGE =
['voltage', 'pf', 'frequency']. Fixes #281.  Code needs a bit of
tidying up!
Merge branch 'master' of github.com:nilmtk/nilmtk

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.