bugfixes and syntax improvemtns

if more than 14 digits are given don round in general,
but test for <eps only.
refine before extrusion
support polyhedron faces

which replaces polyhedron faces
importCSG parse hexfloats in multmatrix parameter
importCSG round rotations
don't call Draft.makecircle and makeprism as they trigger a recompute
use group placholder for objects with no children
OpenSCAD: move removesubtree to OpenSCADUtils
prepare to support the offset operation
add resize keyword
add OpenSCAD_ExplodeGroup command (without icon)
only explode top level objects