viscosityModels::Casson: New viscosity model

Description
    An incompressible Casson non-Newtonian viscosity model.

    References:
    \verbatim
        Casson, N. (1959).
        Rheology of disperse systems.
        In Proceedings of a Conference Organized by the
        British Society of Rheology.
        Pergamon Press, New York.

        Fournier, R. L. (2011).
        Basic transport phenomena in biomedical engineering.
        CRC Press.
    \endverbatim

Contributed by Sergey Sindeev