Rename parameters for Shift and Scale models

The (very basic) [`Shift`](http://docs.astropy.org/en/stable/api/astropy.modeling.functional_models.Shift.html#astropy.modeling.functional_models.Shift) model takes one parameter called `offsets`.  The plural name was to indicate that it could take a list of offsets, and that calling the shift model on an array-like input could shift each element in the array by a different offset.  For example:

```python
>>> s = Shift([1, 2, 3])
>>> s([4, 5, 6])
array([ 5.,  7.,  9.])
```

Previously this capability was a feature only supported in some "special" cases like the `Shift` and `Scale` models.  But since #2634 that sort of capability was generalized to all models in the framework.  So it doesn't make sense to single out these two models, or to pluralize the names of their parameters (since none of the other models use plurals).

I can add backward-compatibility for the old names if needed, but I don't know how much code is relying on this yet.


Also added a couple fairly trivial tests for code in these models that wasn't tested directly.
This comment pertained to some older version of this code and is no longer relevant.