Include low energy cutoff for Powerlaw and mixed classes

Especially for the mixed class, this would help with avoiding having a large spike when transitioning from thermal to non-thermal particles for large values of f_nth.

Start by locating the Powerlaw and mixed class implementations and trace how thermal and non-thermal particles are joined, especially for large f_nth values. Determine the intended low-energy cutoff behavior and verify that applying it removes the transition spike without changing unaffected classes.

Written by the indexing model from the issue text.