- 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'.
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