epix_slide_ref_time_values_default misbehaves on datetimes

[brookslogan]

• For dates converted into POSIXct's, diff, used by guess_period, outputs in terms of days, but + and full_seq adding on that period do it in terms of seconds, yielding a ton of bogus ref time values.
• We should check that the full_seq results actually contain all of the original time_values in case there is numeric messiness that can go on... or perhaps use tol = 0. In former approach, probably want a temporary to hold the sorted unique time values.
• (We should also export guess_period and rename ref_time_values args to time_values.)