Initial implementations of model_expression( ) and translate_model( ).
Handle sub-expressions that are not models.
Add some model functions to SModel.
Add reversible_markov_model to SModel.hs
Fix translate_model
Add formula_expression_ref::perform_exp( )
Use perform_exp().
Add plus_gwf_model.
Add unit_model.
Basic model_expression( ) usage in smodel/setup.C
Make TN+gamma work!
Update HKY and GTR.
Convert HKYx3, TNx3, GTRx3
Convert M0
Update M2a model and some frequencies models.