let the random seed be an optional parameter
Muon production arguments reordered, more tests

muons:
- Renamed function n to n_exponent to avoid variable name conflict
in other functions.
- Reordered arguments to P_mu_total to be more consistent with other
production rate functions. Nuclide object should come before
elevation.

nuclides:
- Change production rate constant names to be more accurate.

Added production rate tests.

Cleaned up some examples.