clean up sfepy/fem/variables.py, add module docstring
new Variables.get_dual_names()
clean up sfepy/fem/equations.py, add module docstring
new Equations.get_variable_dependencies()
new sfepy/base/resolve_deps.py - functions for resolving dependencies

- new get_nums(), solvable(), remove_known(), try_block(), resolve()
update tests/test_base.py: new test_resolve_deps()
fix Equations.get_variable_dependencies() - do not include parameter variables
new Equations.create_subequations()
new ProblemDefinition.create_subproblem()
new EquationSequenceSolver for stationary problems with an equation sequence

- new replace_virtuals()
new examples/thermo_elasticity/thermo_elasticity_ess.py + test
add custom view for thermo_elasticity_ess.py example to script/gen_gallery.py
fix typo in docstring
add custom view for elastic_contact_sphere.py example to script/gen_gallery.py