Make Wp a PionDecay property and clarify that n_H = 1 is assumed

I was running `PionDecay` spectrum computations in a loop and it was annoying to get the log messages about `Wp`. Here I've moved `Wp` to a property so that the user can decide when to compute and print it ... it could also go in a standalone function.

The other change here is that I've documented that `n_H = 1 cm^-3` is assumed in the photon spectrum computation (is that right?). Maybe this should become a parameter of the class with that default value, just like e.g. `B` is a parameter for `Synchrotron`?