update examples/linear_elasticity/linear_viscoelastic.py
update examples/linear_elasticity/linear_viscoelastic.py

- save in HDF5 format
- new post_process() - compute strain, stresses
- update linear_tension()
- new main() - plot time histories

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