insulatingLayer & rsi: NameError/TypeError on massless layers + wrong RMIN/DMIN/KMAX thresholds (v0.9.0)

[phylroy]

Three related bugs in osut 0.9.0, found while building a Python port of the TBD
gem on top of pyOSut. Each makes pyOSut diverge from the Ruby osut behaviour
(and two of them crash) for constructions containing a MasslessOpaqueMaterial,
which are extremely common.

Environment: osut 0.9.0, openstudio 3.11.0, Python 3.12.

1. insulatingLayer — NameError on a massless layer

osut/osut.py:579 (massless branch):

res["r"] = m.thermalResistance()

The loop variable is l, not m, so m is undefined and any construction whose
insulating layer is a MasslessOpaqueMaterial raises
NameError: name 'm' is not defined.
Fix: m → l.

2. insulatingLayer — hardcoded thresholds instead of RMIN/DMIN/KMAX

The qualifying tests are hardcoded as < 0.001 (thermalResistance), < 0.003
(thickness) and > 3.0 (conductivity), whereas Ruby osut uses the constants
RMIN (0.005), DMIN (0.01) and KMAX (2.0). This selects a different
insulating layer than the Ruby gem. Example: a ~0.15 m normalweight-concrete floor
(k ≈ 2.31) is excluded by Ruby (k > KMAX = 2.0) but kept by pyOSut (k > 3.0),
which then changes any downstream U-factor / derating result for that surface.
Fix: use CN.RMIN / CN.DMIN / CN.KMAX (matching osut.rb).

3. rsi — accumulates an Optional instead of a resistance

osut/osut.py:536 (massless branch):

rsi += m.to_MasslessOpaqueMaterial()

Missing .get().thermalResistance(), so rsi raises
TypeError: unsupported operand type(s) for +=: 'float' and 'OptionalMasslessOpaqueMaterial'
for any construction with a massless layer.
Fix: rsi += m.to_MasslessOpaqueMaterial().get().thermalResistance().

Repro

import openstudio
from osut import osut
m = openstudio.model.Model()
lc = openstudio.model.Construction(m)
lc.insertLayer(0, openstudio.model.MasslessOpaqueMaterial(m, "Smooth", 2.0))
osut.insulatingLayer(lc)   # -> NameError (bug 1)
osut.rsi(lc, 0.15)         # -> TypeError (bug 3)

Downstream tracking issue (with the drop-in fixes used as a temporary shim):
canmet-energy/py-tbd#1

Happy to open a PR with the three one-line fixes if that's helpful.