making a start on new algorithm for proportion_of_energy_submetered.
Doesn't run yet.
tweaking comments
New algorithm for proportion_of_energy_submetered works.  But I
suspect i can make it prettier...
Fixing bug when multiple rows from cache match section.start
Tidier MeterGroup.proportion_of_energy_submetered.  But we have an new
bug: we don't always get data from cache.
fix bug caused by `timeframe == pd.NaT` always returning False.
I think MeterGroup.proportion_of_energy_submetered now works.  Some
more tests needed!
Renamed `Electric.proportion_of_mains_energy` to `proportion_of_mains`.
Added simple test for `Electric.proportion_of_energy()`
Merge branch 'proportion_energy_submetered'