ExteriorWall represents heat conduction and heat storage
  within walls. It links a variable number n of thermal resistances
  and capacities to a series connection. n thus defines the spatial
  discretization of thermal effects within the wall. All effects are considered
  as one-dimensional normal to the wall's surface. This model is thought
  for exterior wall elements that contribute to heat transfer to the outdoor.
  The RC-chain is defined via a vector of capacities CExt[n] and a
  vector of resistances RExt[n]. Resistances and capacities are
  connected alternately, starting with the first resistance RExt[1],
  from heat port_a to heat port_b. RExtRem
  is the resistance between the last capacity CExt[end] and the
  heat port_b.
