Center Does Not Work as Expected with 2D Objects

Consider the following:

```python
el = cq.Edge.makeEllipse(x_radius = 1.0, y_radius = 10.0)
print(el.Center())
```

You would expect the center to be `(0, 0, 0)` because the ellipse is centered around the origin.

However, the result is:

```python
Vector: (-1.0993893973079381e-16, -0.1653496405078786, 0.0)
```

I created a branch [here](https://github.com/CadQuery/cadquery/tree/center-tol) which added the tolerance parameter in an effort to correct this. It did not work.

Per @adam-urbanczyk it seems like the class `BRepGProp_Cinert` does the actual work, and does not have a tolerance attribute. So we need to figure out the proper way to handle the fact that `Center()` works fine with 3D objects but not with 2D ones.