Move weighting of zone factors out of Template

Created by: PRemmen

Currently we weight some values in the zone template e.g.:

This should be done in the zone class itself.

##get the coefficient of absorption
<%def name="get_awin(list)", filter="trim">
  <%
  awin = 0.
  sum_area = 0.
  for window in list:
    awin += float(window.layer[-1].material.solar_absorp * window.area)
    sum_area += window.area
  awin_weighted = awin/sum_area
  return str(awin_weighted)
  %>
</%def>