Calculate overall fenestration product U-factor, SHGC, and VT without running simulation

Enhancement Request

Provide SDK users with functionality to access overall fenestration U-factor, SHGC, and VT ... similar to this (open) new feature EnergyPlus request).

Detailed Description

When linking fenestrated subsurfaces (e.g. GlassDoor, FixedWindow) with frame & divider (F&D) objects, there is no obvious solution for users to retrieve the overall fenestration U-factor (as well as SHGC & VT), similar to NFRC calculations. A given building design may have e.g. 3 fenestrated subsurface multilayered constructions, and 17 unique window sizes. The generic nature of F&D objects (i.e. a single F&D object may be reused for multiple fenestrated subsurfaces with unique geometries) offers OpenStudio/EnergyPlus users a very useful (automated) solution to generate a wide range of subsurface-specific properties (great). But in the end, users cannot retrieve the resulting NRFC-like aggregate metrics like overall U-factors. This is becoming an issue for pre-simulation input validation (and/or post-simulation output validation) for code compliance purposes in cold climates e.g., recent codes, upcoming revisions (currently in development).

Possible Implementation

2 options:

1. (preferred) Implement the NFRC/LBNL Window calculations in the OpenStudio SDK to generate subsurface-specific U-factors (standard winter conditions), SHGC (standard summer conditions) ... and VT (less of a priority). A first prototype could be an OpenStudio Measure (or a common gem).
2. (quick fix) If users have F&D inputs (LBNL Window) for a given fenestration product, they likely already have on hand NFRC U-factors for standard winter conditions. Allow SDK users to set a fenestrated subsurface uFactor as a placeholder for pre-simulation input validation (currently not allowed). This would not provide the desired flexibility described above, but it would be better than nothing. EDIT: straightforward to implement with AdditionalProperties as per @shorowit 's suggestion (see below).

Both options would have no bearing on EnergyPlus inputs and/or simulation results.

Start with src/model/SubSurface.cpp and the linked PlanarSurface uFactor API, including the current restriction on setting it. Review the proposed SDK NFRC calculation path against the AdditionalProperties placeholder option and clarify which scope is wanted. Done should expose the requested overall metrics or support pre-simulation placeholders without changing EnergyPlus inputs or simulation results.