Move the following features from:

1. Basic neuron manipulations: https://github.com/natverse/elmr

• Simplify neurons -> https://github.com/natverse/elmr/blob/master/R/simplify_neuron.R
• Stitch neurons -> https://github.com/natverse/elmr/blob/master/R/stitch_neuron.R
• Distal to -> http://natverse.org/elmr/reference/distal_to.html
• prune_twigs http://natverse.org/elmr/reference/prune_twigs.html

2. From registrations: (tbc @jefferis )

• tpsreg -> http://natverse.org/elmr/reference/tpsreg.html
  Each feature must be accompanied by relevant test cases (either synthetic or from actual neuron data)