`QgsPolyhedralSurface` inherits from `QgsSurface` which implements
`QgsAbstractgeometry::isValid` by calling `isValid` from
GEOS. However, GEOS does not handle polyhedral surface. This means
that `isValid` will always return False even if the polyhedral surface
is valid.
This issue is fixed by implementing
`QgsPolyhedralSurface::isValid`. It checks that all the polygons of
the polyhedral surface are valid.
