Added implementation docs on regions and timeperiods to sectormodel
Added implementation docs on regions and timeperiods to sectormodel

tomalrussell/smif,nismod/smif,nismod/smif,willu47/smif,willu47/smif,willu47/smif,tomalrussell/smif,willu47/smif,nismod/smif,tomalrussell/smif,nismod/smif,tomalrussell/smif