ENH: relocate zero_one to pTraits, allows reuse for scalar clamping

ENH: add pTraits and IO for std::int8_t

STYLE: cull some implicitly available includes

- pTraits.H is included by label/scalar etc
- zero.H is included by UList

STYLE: cull redundant forward declarations for Istream/Ostream

src/functionObjects/field/extractEulerianParticles/eulerianParticle/eulerianParticle.H

@@ -49,16 +49,13 @@ SourceFiles
 
 namespace Foam
 {
-// Forward Declarations
-class Istream;
-class Ostream;
-
 namespace functionObjects
 {
+    // Forward Declarations
     class eulerianParticle;
 }
 
-// Forward declaration of friend functions and operators
+// Forward Declarations
 Istream& operator>>(Istream&, functionObjects::eulerianParticle&);
 Ostream& operator<<(Ostream&, const functionObjects::eulerianParticle&);
 

src/functionObjects/field/fieldAverage/fieldAverageItem/fieldAverageItem.H

@@ -92,15 +92,13 @@ SourceFiles
 namespace Foam
 {
 
-// Forward declaration of classes
-class Istream;
-class Ostream;
+// Forward Declarations
 class objectRegistry;
 
 namespace functionObjects
 {
 
-// Forward declaration of friend functions and operators
+// Forward Declarations
 class fieldAverageItem;
 Istream& operator>>(Istream&, fieldAverageItem&);
 Ostream& operator<<(Ostream&, const fieldAverageItem&);

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.C

@@ -670,7 +670,7 @@ Foam::functionObjects::fieldValues::surfaceFieldValue::processValues
             // Uniformity index
             const scalar ui = 1 - numer/(2*mag(mean*areaTotal) + ROOTVSMALL);
 
-            return clamp(ui, 0, 1);
+            return clamp(ui, zero_one{});
         }
 
         default:
@@ -756,7 +756,7 @@ Foam::functionObjects::fieldValues::surfaceFieldValue::processValues
             // Uniformity index
             const scalar ui = 1 - numer/(2*mag(mean*areaTotal) + ROOTVSMALL);
 
-            return vector(clamp(ui, 0, 1), 0, 0);
+            return vector(clamp(ui, zero_one{}), 0, 0);
         }
 
         default:

src/functionObjects/field/stabilityBlendingFactor/stabilityBlendingFactor.H

@@ -152,7 +152,7 @@ Description
     For option 6, the following relation is used:
 
     \f[
-        fCoWeight = clamp((Co - Co1)/(Co2 - Co1), 0, 1)
+        fCoWeight = clamp((Co - Co1)/(Co2 - Co1), zero_one{});
     \f]
 
     where