Coordinate order

`Points` (backed by `matplotlib`) uses coordinates in x, y order. `get_centroids` outputs points in row, column (y, x) order and the `napari`-based tools also use row, column order.

Whichever convention we use we need to have the tools do the conversion as necessary. The order is easy to flip if needed:

```python
new_coords = [(x, y) for y, x in coords]

```