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

- replace with plusOp for reductions. The old use didn't always work
  well with compiler resolution in all cases.
  Simplify touse plusOp, and mark the old version as 'delete'.
  This doesn't not affect very much code.

COMP: incorrect include ordering for GeometricFieldFunctions

- header was included after the template code

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