COMP: remove 'special purpose' minMaxOp (#3326)

- replace with plusOp for reductions. The old use didn't necessarily work
  well with compiler resolution in all cases.
  Simplify to use plusOp (and removed the old version).
  [Does not affect very much code...]

COMP: incorrect include ordering for GeometricFieldFunctions

- header was included after the template code

REGRESSION: combineAllGather mapped to incorrect method (81fa7d08ee)

STYLE: use UPstream::commWarn(...) setter method

src/functionObjects/field/binField/binModels/singleDirectionUniformBin/singleDirectionUniformBin.C

@@ -75,7 +75,7 @@ void Foam::binModels::singleDirectionUniformBin::initialise()
         }
 
         // Globally consistent
-        reduce(geomLimits, minMaxOp<scalar>());
+        reduce(geomLimits, sumOp<scalarMinMax>());
 
         if (!geomLimits.good())
         {