small tweaks in stokes_slip_bc.py example, reference stokes_slip_bc_penalty.py
small tweaks in stokes_slip_bc.py example, reference stokes_slip_bc_penalty.py

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