REF: move get_freq_group to libfreqs, de-duplicate with Resolution.get_freq_group
Having both libresolution.get_freq_group and libresolution.Resolution.get_freq_group is confusing, this adds 1 line to the module-level function to make it handle the method, removes the method, and moves the function to libfrequencies, which is a better fit.
can you rebase rebased+green