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...).
"simplify" might be more appropriate as it results in taking something like 67s and makes it 1m7s.