INT: S-A turbulence model - added enhanced DDES shielding function

Activated using the [optional] 'shielding' keyword having the options:

- standard: [default] same as v2406 and earlier
- ZDES2020: new option based on the reference:

    Deck, S., Renard, N. (2020).
    Towards an enhanced protection of attached boundary layers in hybrid
    RANS/LES methods.
    Journal of Computational Physics, 400, 108970.
    DOI:10.1016/j.jcp.2019.108970

Contribution from Marian Fuchs at UpstreamCFD GmbH

Integrated by Andrew Heather OpenCFD Ltd.