Potential bug for beta_n #33

I think that beta_n is off by a factor of 3/2. It is being computed from the volume average of W_tot instead of P (where Pressure per m^3 is n_eT_e+n_iT_i, but W_tot per m^3 is 3/2 (n_eT_e+n_iT_i), but the docstring in the W_tot def. says they're equal.) Fixing this makes my popcon runs match ITER and MANTA beta_n values much better. Let me know if I'm missing something, and if not, I'm happy to implement a fix @matt-pharr.

Beta_n def: