Default NetCDF dimensionless unit.

This PR corrects the NetCDF loader such that it honours the CF convention that states quantities with no `units` are deemed as `dimensionless`, rather than `unknown` as currently implemented i.e.

```
Units are not required for dimensionless quantities. A variable with no units attribute is assumed to be
dimensionless. However, a units attribute specifying a dimensionless unit may optionally be included.
```

See http://cf-convention.github.io/1.6.html#units for further clarification.

Fixes #935.