Feature Request: DataSet: implement radius search in bruteforce knearest to match kdtree

### Is your feature request related to a problem? Please describe the problem.

at the moment the knearest interfaces of DSet and KDTree are different. 

### Describe the solution you'd like to see.

Let's make the former full featured to match the latter.

### Describe alternatives you've considered

nothing

### Additional context

we strive for interface consistency