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...).
I know - I was trying to go for something that the least number of people might
code incorrectly. A better name would be appreciated - any thoughts?