Bugfix/symbol normalizer

• When initializing Model, symbol_normalizer was not applied when normalize=True. This was an oversight and resulted in inconsistent internal representations for identical models, especially when Model.subs is used.

• test_Model_subs in test_model.py was changed to remove sensitivity to dict ordering, which caused problems with python=3.5.

• test_Model was changed to test for the correct internal format.

• Random number generator seed set for these tests.