remove examples/diffusion/octahedron.py + test

- no added value w.r.t. other diffusion examples
update examples/linear_elasticity/linear_elastic.py: comments -> docstring
clean up run_tests.py
clean up sfepy/base/testing.py
clean up sfepy/discrete/materials.py
clean up tests/tests_basic.py
clean up sfepy/mechanics/matcoefs.py
Merge pull request #270 from vondrejc/clean_up
new general term