Extend the field_statistics function to handle individual ertbox grid per zone

[oddvarlia]

Individual ertbox grid is now implemented in ERT. When using this the field statistics function need to be extended to handle this. Currently it assumes that ertbox grid is the same for all zones and only defined by the GRID keyword in ERT.

Propose to extend the config file format for field_statistics with a keyword:

ertbox_per_zone:
  <zone_name>: <ertbox_grid_name_for_this_zone>
  <zone_name>: <ertbox_grid_name_for_this_zone>
   ....
  <zone_name>: <ertbox_grid_name_for_this_zone>

And if there exist zones without any specified ertbox, it will look for a file ERTBOX.EGRID under <CONFIG_PATH>/../../rms/output/aps and assume this is the definition that should be used.

The configuration file must be consistent with the ERT config file regarding field parameters and ertbox grids.