Incorrectly subtracting R-5 when using ExpandedPolystyreneSheathing element

[nmerket]

If you have a wall like this

<Wall>
  <SystemIdentifier id="wall_right"/>
  <WallType>
    <WoodStud>
      <ExpandedPolystyreneSheathing>true</ExpandedPolystyreneSheathing>
    </WoodStud>
  </WallType>
  <Orientation>east</Orientation>
  <Siding>wood siding</Siding>
  <Insulation>
    <SystemIdentifier id="wall_right_insulation"/>
    <Layer>
      <NominalRValue>0</NominalRValue>
    </Layer>
  </Insulation>
</Wall>

This code

hescore-hpxml/hescorehpxml/base.py

Lines 186 to 190 in e5af845

	if tobool(xpath(hpxmlwall, 'h:WallType/h:WoodStud/h:ExpandedPolystyreneSheathing/text()')) or has_rigid_ins:
	wallconstype = 'ps'
	# account for the rigid foam sheathing in the construction code
	wall_rvalue -= 5
	rvalue = wall_round_to_nearest(wall_rvalue, (0, 3, 7, 11, 13, 15, 19, 21))

makes the R-value go negative and causes an error.

The ExpandedPolystyreneSheathing element is kind of weird and should probably be removed from HPXML because the same thing can be described better with an Insulation/Layer. There's no R-value associated with it, so it's not clear whether we're to assume one or expect one in a layer below. Right now the translator apparently expects the R-value to be present in a layer, hence the error.

We could just check if wall_rvalue goes negative and set it to zero if it does. It's kind of a kludgy fix, but avoids the error.

[nmerket]

See also the docs for this part of the translation.

[nmerket]

Verify that it's being handled in #160.