ENH: additional contiguous traits (#1378)

- change contiguous from a series of global functions to separate
  templated traits classes:

    - is_contiguous
    - is_contiguous_label
    - is_contiguous_scalar

  The static constexpr 'value' and a constexpr conversion operator
  allow use in template expressions.  The change also makes it much
  easier to define general traits and to inherit from them.

  The is_contiguous_label and is_contiguous_scalar are special traits
  for handling data of homogeneous components of the respective types.

src/meshTools/cellClassification/cellInfo.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           |
+    \\  /    A nd           | Copyright (C) 2019 OpenCFD Ltd.
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
                             | Copyright (C) 2011-2016 OpenFOAM Foundation
@@ -47,12 +47,10 @@ SourceFiles
 
 namespace Foam
 {
+
+// Forward Declarations
 class polyPatch;
 class polyMesh;
-
-
-// Forward declaration of friend functions and operators
-
 class cellInfo;
 
 Istream& operator>>(Istream&, cellInfo&);
@@ -69,6 +67,7 @@ class cellInfo
 
         label type_;
 
+
     // Private Member Functions
 
         //- Update current cell/face type with neighbouring
@@ -212,12 +211,13 @@ public:
 };
 
 
-//- Data associated with cellInfo type are contiguous
-template<>
-inline bool contiguous<cellInfo>()
-{
-    return true;
-}
+// * * * * * * * * * * * * * * * * * Traits  * * * * * * * * * * * * * * * * //
+
+//- Contiguous data for cellInfo
+template<> struct is_contiguous<cellInfo> : std::true_type {};
+
+//- Contiguous label data for cellInfo
+template<> struct is_contiguous_label<cellInfo> : std::true_type {};
 
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //