update test_invariance_qp() in tests/test_mesh_interp.py - test field gradients
update test_invariance_qp() in tests/test_mesh_interp.py - test field gradients

- increase field approximation order to 2

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