integrator.h dict key may not exist but is forced to in python for now

[PhilipDeegan]

the key "refinement" may not exist if not populated from python https://github.com/PHAREHUB/PHARE/blob/master/src/amr/wrappers/integrator.h#L106

hence we force a group addition so we can count on the key exising https://github.com/PHAREHUB/PHARE/blob/pyphare/pyphare/pharein/init.py#147

follow on from #477