Integrate decay mode for isotope dose rate calculations

[leloup314]

Currently, the dose rate calculations for isotopes, performed using the isotope_dose_rate function in irrad_spectroscopy.physics, do not respect the decay mode with which the given isotope decays. This results in an underestimation of the dose rate for e.g. cases where a beta plus (positron) decay generates 2 additional 511 keV photons.

This is partially related to #11, where the information on the decay mode for many isotopes is incorrect or outdated