Re-worded the description for `site_meter` to hopefully make it clearer what to do in multiple-phase setups and, also, in setups (like in UK-DALE) where there are redundant site meters.  Also added `phase` attribute.  Any thoughts, @nipunreddevil?
Re-worded the description for `site_meter` to hopefully make it
clearer what to do in multiple-phase setups and, also, in setups (like
in UK-DALE) where there are redundant site meters.  Also added `phase`
attribute.  Any thoughts, @nipunreddevil?

nilmtk/nilm_metadata,AlexRobson/nilm_metadata,nilmtk/nilm_metadata