Implement decimalyear format in time module

@justincely and I implemented the format decimalyear in the astropy time module.  
I think this can be done much more efficiently (in a vectorized way) without the Python `datetime` object.  Roughly speaking:

- Compute `iy_start = int(val)` and `iy_end = iy_end + 1`
- Use `erfa_time.dtf_jd` to compute the corresponding `jd1` and `jd2` values for start and end year.  
- Create `Time` objects `t_start` and `t_end` using those `jd1_start`, `jd2_start` and `jd1_end`, `jd2_end` values and the given scale and with `format='jd'`
- Compute the fractional times as `t_frac = t_start + (t_end - t_stop) * frac`
- Set the `self.jd1 = t_frac.jd1` and `self.jd2 = t_frac.jd2`

Hopefully something close to this should work and I haven't missed something.  @mhvk - does this look about right?