`time_string` as implemented for TAI is not a string anymore.  How do we solve this?

If we leave it as it is, and I follow the docs, I would do:
```python
tai_time = parse_time('-1121232423.89', tai = True)
```
and it will fail.  I think it will fail too for integer time.  I suggest we update the docstring to say `time_string` is either a `str`, a `datetime` object or a number.