minor cleanup of turbulence models

- added constructor dictionary(const dictionary*) that also handles NULL
  pointers and makes it convenient to construct from a possibly nonexistent
  sub-dictionary:
  eg,
      dictionary dict2(dict1.subDictPtr("someDict"));

- make some of the turbulence Coeffs sub-dictionary optional.
  Their contents are all 'lookupOrAddDefault' anyhow.

- in turbulentMixingLength BCs, skip namespace qualifier in template
  (eg, <RASModel> vs. <compressible::RASModel>)

- change comments from 'turbulenceProperties' to RASProperties/LESProperties

- consistency between compressible/incompressible - no separate file for
  'New' selector etc

- consistency in accessing the model coefficients.
  Use method coeffDict() for const access.
  Use protected data member coeffDict_ for read/write access.

- document model coefficients in etc/constant/RASProperties.
  Need the same for LESProperties before we can prune these from the
  tutorials.

applications/test/dictionary/dictionaryTest.C

@@ -52,10 +52,17 @@ int main(int argc, char *argv[])
 
         Info<< "dict1.toc(): " << dict1.name() << " " << dict1.toc() << nl
             << "dict2.toc(): " << dict2.name() << " " << dict2.toc() << endl;
-        
+
         // copy back
         dict1 = dict2;
         Info<< "dict1.toc(): " << dict1.name() << " " << dict1.toc() << endl;
+
+        dictionary dict3(dict2.subDictPtr("boundaryField"));
+        dictionary dict4(dict2.subDictPtr("NONEXISTENT"));
+
+        Info<< "dictionary construct from pointer" << nl
+            << "ok = " << dict3.name() << " " << dict3.toc() << nl
+            << "no = " << dict4.name() << " " << dict4.toc() << endl;
     }