make overlayCnvCntrs return contour object

Made overlayCnvCntrs return the plotted contour object. This way, it can be removed like this:

```python
# plot
mapDatObj = pydarn.plotting.plotMapGrd.MapConv(datetime, mapObj, ax)
contour = mapDatObj.overlayCnvCntrs()

# remove
for x in contour.collections: x.remove()
for x in contour.labelTexts: x.remove()
```

If one's creating an animation/video, removing per-frame elements is an important ability.