ENH: extend VectorSpace traits to include pTraits_cmptType

- The pTraits_cmptType returns the data type of 'cmptType' (for
  arithmetic and VectorSpace types) or is simply a pass-through.

  This can be combined with the pTraits_nComponents for casting.
  For example,

  function
  (
      reinterpret_cast<pTraits_cmptType<Type>::type*>(buf.data()),
      (buf.size()/pTraits_nComponents<Type>::value)
  );

ENH: extend Foam::identityOp so support array indexing (pass-through)

applications/test/UList/Test-UList.C

@@ -115,7 +115,22 @@ int main(int argc, char *argv[])
         Info<<"UList: "; print(ulist);
     }
 
+    {
+        Foam::identityOp ident;
+
+        Info<< nl << "identityOp as functor or array/map" << nl;
+
+        for (const label val : labelRange(5))
+        {
+            Info<< "value:" << val
+                << " () = " << ident(val)
+                << " [] = " << ident[val] << nl;
+        }
+    }
+
+    Info<< "\nEnd\n" << nl;
     return 0;
 }
 
+
 // ************************************************************************* //

applications/test/pTraits/Test-pTraits.C

@@ -34,6 +34,7 @@ Description
 #include "boolVector.H"  // A FixedList pretending to be a vector
 #include "vector.H"
 #include "tensor.H"
+#include "complex.H"
 #include "uLabel.H"
 #include "Switch.H"
 
@@ -110,6 +111,7 @@ void printTraits()
         << " vector-space=" << Switch::name(is_vectorspace<T>::value)
         << " is_label=" << Switch::name(is_contiguous_label<T>::value)
         << " is_scalar=" << Switch::name(is_contiguous_scalar<T>::value)
+        << " cmptType=" << typeid(typename pTraits_cmptType<T>::type).name()
         << endl;
 }
 
@@ -120,6 +122,12 @@ void printTraits(const pTraits<T>& p)
     Info<< p.typeName << " == " << p << endl;
 }
 
+template<class T>
+void printDecltype()
+{
+    Info<< "cmptType : " << typeid(T).name() << nl;
+}
+
 
 #pragma GCC diagnostic warning "-Wmaybe-uninitialized"
 #pragma GCC diagnostic warning "-Wuninitialized"
@@ -129,12 +137,16 @@ int main()
     printTraits<bool>();
     printTraits<label>();
     printTraits<scalar>();
-    printTraits<vector>();
+    printTraits<complex>();     // Uses specialized pTraits_...
+    printTraits<floatVector>();
+    printTraits<doubleVector>();
     printTraits<tensor>();
-    printTraits<boolVector>();
+    printTraits<boolVector>();  // Uses specialized pTraits_...
     printTraits<word>();
     printTraits<std::string>();
 
+    Info<< nl;
+
     {
         pTraits<bool> b(true);
         printTraits(b);
@@ -147,6 +159,8 @@ int main()
 
     printTraits(pTraits<scalar>(3.14159));
 
+    Info<< nl;
+
     label abc;
     Info<< "uninitialized primitive:"<< abc << endl;