switch NSE to use constexpr_for on bion and mion

The way `bion` and `mion` are implemented in the networks is via a loop over a `constexpr` function templated on the species index.  In the self-consistent NSE solver, we are using these legacy interfaces to `bion` and `mion` which take O(N) work to index.  We should switch those loops to `constexpr_for` and use the templated interface to `mion` and `bion`.