Attempt to calculate delta_ut1_utc when the attribute is accessed directly

A partial answer to the bugs reported in #1924. Now:
```
from astropy.time import Time
t = Time('2010-02-03 12:13:14', scale='utc')
t.delta_ut1_utc
```
yields
```
array([ 0.08327665])
```
instead of giving a rather obscure error message