move around the detector response to the signal into a new function

See merge request Monash/peyote!15

# Conflicts:
#	peyote/detector.py
#	peyote/likelihood.py
#	tutorials/BasicTutorial.py
#	tutorials/standard_data.txt

Moritz Huebner: Some simplifications in the initialise_parameters() method and turned around how keys are being checked in verify_priors

this requires passing a subset of the parameters to the new function