Radiant slab model update

The current model is not accurate when a material with thermal properties very different than the slab is included in the construction.
Typically, this layer would be an insulation layer (conductivity much lower than that of slab material).
This commit allows for the user to specify the insulation layer in the construction.  All layers "below" that layer are ignored when calculating the 2D conduction.
An extra parameter is required in the plant database.  This will be added soon in the Databases directory.
The extra parameter can be set to zero if the user does not want to specify an insulation layer.  If the parameter is set to zero, the solution will be the same as before the changes to the code.