TimeInterval class: add get_seconds method and input prettifying

This adds a `get_seconds` method to get the total number of seconds
represented in a `TimeInterval`. It also adds a `standardize`
constructor keyword argument that switches on adjusting inputs into a
nice form.

@arjclark, please review (and test the timepoint tests...).
Any reason for separating date from time? Could these operations be refactored into a single one? It feels like you've gone most of the way towards this in the change you've made.