Change how generate_maps (grid3d_aggregate_maps.py) handles undefined values

[vegardkv]

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.