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/functionObjects/field/nearWallFields/findCellParticle.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) 2013-2017 OpenFOAM Foundation
@@ -45,11 +45,8 @@ SourceFiles
 namespace Foam
 {
 
+// Forward Declarations
 class findCellParticleCloud;
-
-
-// Forward declaration of friend functions and operators
-
 class findCellParticle;
 
 Ostream& operator<<(Ostream&, const findCellParticle&);
@@ -273,11 +270,10 @@ public:
 };
 
 
-template<>
-inline bool contiguous<findCellParticle>()
-{
-    return true;
-}
+// * * * * * * * * * * * * * * * * * Traits  * * * * * * * * * * * * * * * * //
+
+//- Contiguous data for findCellParticle
+template<> struct is_contiguous<findCellParticle> : std::true_type {};
 
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //