determine default --position-vector from mesh bounding box in pv_plot()
determine default --position-vector from mesh bounding box in pv_plot()

- set --position-vector default to None in main()
- fixes placement of plots for non-3D meshes

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