Issue408 refrigerants r134a r410a

Created by: MirkoEngelpracht

Hello everybody, the issue is solved and ready to be reviewed.

I've added a few examples and validation models. Within the validation models, I've checked all medium functions including the fitted formulas as well as the derivatives. Furthermore, I've compared the results to external media libraries, i.e. the HelmholtzMedia library and ExternalMedia library.

Actually, some problems occur while using partial derivatives with respect to specific entropy. Moreover, I'm not quite satisfied with my solution of implementing a general form of two-dimensional polynomials as done, for example, for the function 'density_pT'. I've also added the coefficients of fitted formulas directly in the model for reasons of simulation speed.

Perhaps somebody has some ideas to provide a nicer solution for the issues mentioned above.

Best regards Mirko