- 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'.