DOC: elaborate the usage of function objects

ENH: update libs of etc/caseDicts/postProcess items
  ENH: ensure destructor=default
  ENH: ensure constness
  ENH: ensure no 'copy construct' and 'no copy assignment' exist
  TUT: add examples of function objects with full set
       of settings into a TUT if unavailable
  TUT: update pisoFoam/RAS/cavity tutorial in terms of usage

src/OpenFOAM/db/functionObjects/writeFile/writeFile.C

@@ -214,7 +214,12 @@ Foam::functionObjects::writeFile::writeFile
 bool Foam::functionObjects::writeFile::read(const dictionary& dict)
 {
     writePrecision_ =
-        dict.getOrDefault("writePrecision", IOstream::defaultPrecision());
+        dict.getCheckOrDefault
+        (
+            "writePrecision",
+            IOstream::defaultPrecision(),
+            labelMinMax::ge(0)
+        );
 
     updateHeader_ =
         dict.lookupOrDefault("updateHeader", updateHeader_);