DoB from chi

[Moohan]

To close #42.

There was a discussion recently about this again within my team, so I've had a first stab at a generic function.

Still to do are tests and documentation.

Happy to receive any comments on the interface for this.

The idea I've gone with is that the users give some chi numbers and optionally a min and max-age (e.g. my dataset is children so 0 - 18) and a min and max date (e.g. date of the episode would be a 'max date' as they can't be born before that, min date is really an alternative to max age).

[Moohan]

I think this is ready for review now. I still need to:

• [ ] Add some tests for age_from_chi.
• [ ] Add some links to the Readme / make a vignette.