Import of InstrumentResponse from threeML is incorrect

[dsvinkin]

Hi all!
The import statement in drmgen.py
from threeML.utils.response import InstrumentResponse
should be
from threeML.utils.OGIP.response import InstrumentResponse
to correspond of the current structure of threeML.

[grburgess]

thanks! If you would, could you make fork and do a PR? I've also separated the the response stuff out of 3ML to bypass that as a dependency, but this should be fixed.

[grburgess]

Thanks for fixing this @dsvinkin