- in continuation of #2565 (rotationCentre for surface output formats)
  it is helpful to also support READ_IF_PRESENT behaviour for the
  'origin' keyword.

  This can be safely used wherever the coordinate system definition
  is embedded within a sub-dictionary scope.

  Eg,
      dict1
      {
          coordinateSystem
          {
              origin (0 0 0);  // now optional here
              rotation ...;
          }
      }

   but remains mandatory if constructed without a sub-dict:

      dict2
      {
          origin (0 0 0);   // still mandatory
          e1  (1 0 0);
          e3  (0 0 1);
      }

   With this change, the "transform" sub-dictionary can written
   more naturally:

       formatOptions
       {
           vtk
           {
               scale 1000;  // m -> mm
               transform
               {
                   rotationCentre  (1 0 0);
                   rotation axisAngle;
                   axis    (0 0 1);
                   angle   -45;
               }
           }
       }

ENH: simplify handling of "coordinateSystem" dictionary lookups

- coordinateSystems::NewIfPresent method for optional entries:

    coordSysPtr_ = coordinateSystem::NewIfPresent(mesh, dict);

  Instead of

    if (dict.found(coordinateSystem::typeName, keyType::LITERAL))
    {
        coordSysPtr_ =
            coordinateSystem::New
            (
                mesh_,
                dict,
                coordinateSystem::typeName
            );
    }
    else
    {
        coordSysPtr_.reset();
    }

ENH: more consistent handling of priorities for binModels, forces (#2598)

- if the dictionaries are overspecified, give a 'coordinateSystem'
  entry a higher prioriy than the 'CofR' shortcuts.

  Was previously slightly inconsistent between the different models.