Logic error in ops_bgcon which does not alter the outcome.

[monty241]

The GridValue is assigned the average when isCell is false.

New code matches the specification: "return grid average if point is outside grid or if value is negative".

Generic question: Should GridValue return any value or (even the average of a field as currently) when a value outside the domain is requested? If yes and yes the average, then an alternative might be to for negative value alone and not on whether isCell is true or not.

Changes developed by Gerard Cats.