Add/Modify PreCAD usage and options.

Add/Modify PreCAD usage and options.
The most significant options have dedicated methods:
SetPreCADMergeEdges(bool)
SetPreCADRemoveNanoEdges(bool)
SetPreCADEpsNano(double)
SetPreCADDiscardInput(bool)

The others options can be set using the method SetOption. In this later case the names should be prepend by "PRECAD_".