new gen_extended_block_mesh() mesh generator

- new _spread_along_axis(), _get_extension_side()
- only "do nothing" grading
provide reasonable grading function in gen_extended_block_mesh()

- grading function now accepts x in [0, 1]
- update _spread_along_axis()
- verify merging by checking the number of nodes, update _get_extension_side()
allow to run python shell from debugger
force parsing whole string in dict_from_string()
more grammar elements available in parse_conf.py
move definitions inside functions in parse_conf.py
radial mesh - fixes, integrals and derivatives
clean up sfepy/physics/radial_mesh.py
remove duplicate __call__() in RadialVector
fix docstring of gen_cylinder_mesh()
new tests/test_mesh_generators.py - test basic mesh generators