fix subset_bbox with level_values

[ ] This PR addresses an already opened issue (for bug fixes / features)
- This PR fixes issue #xyz
[x] Tests for the changes have been added (for bug fixes / features)
[ ] Documentation has been added / updated (for bug fixes / features)
[ ] HISTORY.rst has been updated (with summary of main changes)
[ ] I have added my relevant user information to AUTHORS.md
What kind of change does this PR introduce?: Bug fix
Does this PR introduce a breaking change?: no
Other information:

subset_bbox raise an exception when used both with area and level_values parameter:

subset_level_by_values got multiple values for argument level_values

This PR fixes this issue. The parameter keyword level_values was missing needed by the check_levels_exist decorator.

roocs / clisops