Fix numba parallel segfault in `compute_Menv`

[Lehman Garrison]

In #45, we found that using Numba parallel was causing a segfault in compute_Menv and calc_fenv in prepare_sim. It doesn't appear to be a problem with the functions themselves, but rather something at the library level. Probably related to using Numba parallel inside multiprocessing (even though we are using the nominally fork-safe workqueue backend).

Debugging this is probably a heavy lift, so for now we've just disabled Numba parallel for those two functions.