Create a new, independant, model containing all the statements in this model together with all of those in another given model. By
independant we mean that changes to the result model do not affect the operand models, and
vice versa.
The new model need not be of the same type as either this model or the argument model: typically it will be a memory-based model, even if this model is a database model.
@return A new model containing all the statements that are in either model
@param model The other model whose statements are to be included.