ENH: applyBoundaryLayer - optionally write turbulence fields

• The previous option 'write-nut' controlled the writing of turbulence nut, but other turbulence fields were always written. These have been shown to be a source of instability for many cases.

  This commit replaces the 'write-nut' option by a 'writeTurbulenceFields' option that controls the writing of all turbulence fields. If not set, only the velocity field is written.

  For compatibility, the old 'write-nut' option is still recognized but is redirected to 'writeTurbulenceFields'.