If the user supplies a spectral image to the psf system (this is only done
in the cluster fitting at present) then when the data routine is called
energy information must propogate from the client down to that point. This
is achieved using either psf_def or psf_defb. In the first
case the user supplies energy and time bands (the latter are not currently
used anywhere) for psf evaluation. The alternative psf_defb supplies
bin numbers in the energy and time dimensions for psf evaluation. The psf
data routine for an energy dependent psf should check for the energy band psf
instance values before computing the psf. See psf_xrt_pspc in
psfrtn.f for an example. If the psf is energy dependent bu the
user supplies only an image, then the initialisation routine should
prompt for a mean photon energy.