new ContactPlane - contact plane with polygonal boundary
new ContactPlane - contact plane with polygonal boundary

- new sfepy/mechanics/contact_planes.py
- new plot_polygon(), plot_points()

vlukes/sfepy,sfepy/sfepy,vlukes/sfepy,RexFuzzle/sfepy,sfepy/sfepy,rc/sfepy,lokik/sfepy,lokik/sfepy,vlukes/sfepy,sfepy/sfepy,lokik/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,lokik/sfepy,rc/sfepy,BubuLK/sfepy,RexFuzzle/sfepy,rc/sfepy,BubuLK/sfepy,RexFuzzle/sfepy