start support of 'plate' integration/connectivity type
update CMapping - new mtx_t attribute
update Approximation.describe_geometry() for plate integration
update get_physical_qps() - new map_kind argument
update Term.get_physical_qps() for plate integration
update get_shape_kind(), Term.iter_groups() for plate integration
update FieldVariable.get_data_shape(), .evaluate() for plate integration
update Approximation.get_connectivity() for plate integration
new transform_asm_vectors(), transform_asm_matrices()
update TLMembraneTerm.weak_function() - use transform functions
update QuadraturePoints.from_table() to store true order
new sfepy/postprocess/plot_quadrature.py

- new _get_qp(), plot_weighted_points(), plot_quadrature()
new script/plot_quadratures.py - closes #178
fix docstring of VolumeSurfaceTerm
docs: add labels to tutorial
update docstrings of poisson.py, poisson_short_syntax.py examples

- add mutual references, tutorial reference
- remove comments in examples/diffusion/poisson.py as their text is in the
  tutorial
update docstring of poisson_periodic_boundary_condition.py example
update docstring of poisson_field_dependent_material.py example
new examples/diffusion/poisson_neumann.py + test - closes #195
simplify ProblemDefinition.__init__() - re-implements #212