add Constructor to avoid needing to have users add OffsetArrays

[alecloudenback]

E.g. with a slightly customized (e.g. improved) mortality rate, let users create a mort object like:

mortality(vec,start_age=age)

instead of

using OffsetArrays
OffsetArray(vec,age)

Can currently do this with UltimateMortality but it's not clear to user that this is semantically the same as above.