Basic implementation of prune(.dotprops) function

* includes an example
* handling of return.indices could be improved
fix bug in pruning points and add tests
test and fix for returning indices
Add prune.neuronlist method

* including test and docs
Add xyzform.neuronlist method

* closes #96
test pruning a neuronlist with a neuronlist and provide example
Merge branch 'feature/prune'

* feature/prune:
  test pruning a neuronlist with a neuronlist and provide example
  Add xyzform.neuronlist method
  Add prune.neuronlist method
  test and fix for returning indices
  fix bug in pruning points and add tests
  Basic implementation of prune(.dotprops) function

* closes #94