Refactor for nuclide specific scaling

Nuclide specific production rates are now in the nuclide module classes.
These are used by the spallation function P_sp in the production module.

- Started implementing PHITS (Sato 2008).
- Add time parameter to some functions for future time-dependent
  scaling.
- Changed pressure parameter name for Stone2000 scaling from P to
  pressure.
- Add pressure as optional parameter to P_sp and P_tot.
- Made the simulation tests use named parameters, at least until the API
  settles down.
Default to Sato production rates

Make Sato production rates the default, but default to Stone
scaling scheme.