Change how `generate_maps` (`grid3d_aggregate_maps.py`) handles undefined values

Currently, `generate_maps` (of `grid3d_aggregate_maps.py`), can ignore cells with properties below a certain threshold. The primary purpose of this is speed-up. However, this also affects the result: areas with no cell properties above this threshold is handled in the same manner as areas outside the extent of the grid. This can be confusing and also leads to issues when doing surface-surface operations with `xtgeo` (differences maps or statistics).

The default behavior should be the same for this function as it is for the functions under `avghc` (to the extent reasonable). One option could be to add an optional fill value to be used for undefined values, and perform this as a post-processing step before returning `results` in the `generate_maps` function. This should be relatively easy to implement, however it has the limitation that this value will also be used outside the grid extents.

Start in grid3d_aggregate_maps.py at generate_maps and inspect how thresholded cells become undefined before results is returned. Compare the undefined-value behavior of the avghc functions, then determine how undefined cells should remain distinct from areas outside the grid extent. Done means generate_maps follows the agreed default behavior without breaking surface-surface operations.

Written by the indexing model from the issue text.