Grid Calculator: Difference between grid wrong with different active cells

[hhgs]

Sum of delta volumes between grid models becomes wrong when different active cells are used (Grid Calculator or Difference option). In this case, an union of active cells should be used for the grid model where results are appended.

[magnesj]

This will probably require some rework of the way we store and use active cells for a grid model.

Add note to online manual describing the currently supported algorithm.