Absolute time and relative time

ClimaAnalysis currently uses relative time because it is easier to work with for some cases (e.g. resampling and making interpolants). However, there are cases where it is easier to work with dates or periods. For example, you might want to slice time at `Dates.Month(1)` rather than at `86400 * 31` seconds or specify the date you want to slice at.

Start by reviewing the existing relative-time handling used for resampling, interpolants, and slicing. Define how absolute dates or periods and relative seconds should coexist, including cases such as Dates.Month(1) and date-based slicing; done means those workflows are supported.

Written by the indexing model from the issue text.