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

• Tutorial: $FOAM_TUTORIALS/incompressible/pimpleFoam/LES/NACA4412

Contribution from @MarianFuchs at UpstreamCFD GmbH

Integrated by Andrew Heather OpenCFD Ltd.