Allow use of Dates for Time Dimension

[lrennels]

As discussed here: https://forum.mimiframework.org/t/working-with-dates-time-steps/184/2 it would be very useful to allow users to input Dates instead of integers for their time steps. This may be a significant lift for internal structures, but we might be able to depend on Julia's multiple dispatch for array types ... so let's scope it out and see.