Refactored computation of Gocad face sets again.
Added functionality for extracting surfaces for simulation model setup.
Read at most number of cells property values from binary file.
Moved public methods in one connected part and private methods/attributes in one connected part.
[GocadSGridReader] getPropertyVec() -> getProperty(), changed also return value.

Since it is useful to have "no data values" outside of the object, the
GocadProperty  structure is returned. Consequently the vec of prop vals
was moved into struct GocadProperty, too.
Functionality to substitute no data values in properties.
Take over the face set info for split nodes.
Writing the face set mesh  in the same folder as the orig mesh.