Added `_` as shortcut to `dimensionless_unscaled`

I find creating dimensionless `Quantity`es overly painful compared to quantities with dimensions. Currently the two existing options are:

1. Putting `u.km`, `u.s` next to `u.dimensionless_unscaled`. Too large.
2. Putting `u.km`, `u.s` next to `u.Unit('')` or `u.Unit(1)`. Why the inconsistency?

I find dimensionless quantities very convenient, and hence this tiny addition. It's the shortest and least obstrusive way I found to use them.

```
a = 1.0 * u.AU
ecc = 0.0167 * u._
inc = 0.0 * u.deg
...
```