Modifies `decay` module to have default value for n_atoms.
Modifies `decay` module to have default value for n_atoms.

Methods involving `n_atoms` have `n_atoms=None` set as default. In this case
`get_n_atoms` will be called with SNO+ defaults (0.3% loading, 3.5m, etc.) to
set a value for n_atoms.

jwaterfield/echidna,drjeannewilson/echidna,mjmottram/echidna,arushanova/echidna,ashleyrback/echidna,snoplusuk/echidna