ENH: Added new createViewFactors utility

Creates view factors for the view factor radiation model.

User-selectable models:

- raySearchEngine: model to generate rays, i.e. face-to-face connections
- viewFactorModel: model to compute the view factors

For visualisation, use:

- Write the view factors as a volume field

    writeViewFactors    yes;

- Write the rays using OBJ format:

    writeRays       yes; // default = no

Participating patches must be in the \c vewFactorWall group, i.e. using the
\c inGroups entry of the "\<case\>/polyMesh/boundary" file.

\verbatim
myPatch
{
    type            wall;
    inGroups        2(wall viewFactorWall);
    ...
}
\endverbatim

Reads:

- <constant>/viewFactorsDict : main controls
- <constant>/finalAgglom : agglomeration addressing (from faceAgglomerate)

Generates:

- <constant>/F : view factors (matrix)
- <constant>/mapDist : map used for parallel running
- <constant>/globalFaceFaces : face addressing