GEARRT: Change default behaviour of photon frequency bin edges

changed default behaviour of photon bins: don't assume first photon bin to start at 0Hz with the upper limit of the first user provided frequency. Instead, take first user provided frequency as lower limit.

When I first imlemented it, I meant well and had great plans, but this version is more practical. If users want the frequency bin [0, something else) to exist, all they need to do is to specify it in the parameter file and configure with enough frequency bins.