Introduce PixelRegion.overlaps

For some use cases e.g. when placing regions algorithmically a method like `PixelRegion.overlaps(other_region)` would be useful. This will be easily to implement once e.g. `CompoundRegion.area` is supported (see #433). In this case the implementation would reduce to:
```python3
def overlaps(self, other):
intersection = self.intersection(other)
return intersection.area > 0
```
For some specific cases it could also be implemented analytically if needed.

Start by locating PixelRegion and CompoundRegion, then read issue #433 about CompoundRegion.area and inspect the existing intersection behavior. Determine how overlap should handle the region types and boundary cases; done means PixelRegion.overlaps(other_region) reliably reports whether the intersection has positive area.

Written by the indexing model from the issue text.