Consistent circle transform.

Depending on compilation flags, applying a transform used to either:
- work;
- fail throwing an abstract method not implemented message;
- fail silently.

Now it should consistently work, like the other geometries.

Fix #2680 
Minor, but I'd use this for more clarity:

```
The geometry is modified in place. This also means that the transformation doesn't change the type of the geometry: a line is transformed to line, a circle to a circle, etc. If you do not want…
```