Fixed priority bug in print_exp2 and print_exp2_str

Added dump_exp_graphviz
Added exp_contains (can check if a component reference occurs in an expression)
Added solve (can solve linear equations
Added get_terms_containing_x (not used anymore)
Added rules to simplify to simplify expressions (0+a, 1*a, - - a etc)


git-svn-id: https://openmodelica.org/svn/OpenModelica/trunk@1201 f25d12d1-65f4-0310-ae8a-bbce733d8d8e