Add decay date to output

[pkienzle]

Give decay date as y-m-d h:m:s in addition to the amount of time to decay below 0.1 nCi.

Assume time off beam gives the time that the sample was removed from the beam (absolute) or the time since the sample was removed from the beam (relative).

E.g.,

    Time to decay below 0.100 nCi is 55.8 yrs (2072-11-13)
    Time to decay below 0.100 nCi is 21.9 days (2018-02-16 07:42)
    Time to decay below 0.100 nCi is 97.2 min (2018-01-15 17:24)

Probably should include the date/time that the sample was removed from the beam as well.