Implement an SWC exporter for skeletonized neurons

[Marwan Abdellah]

Incrementally, implement a neuron SWC exporter from the skeletonized dataset.
The exporter should:

1. Initially, export all the valid branches as dendrites, even including the spines.
2. Categorize the branches into either axons or dendrites.
3. Recognize the apical dendrites from the basal dendrites.
4. Filter the spines and export them individually.

[Marwan Abdellah]

First implementation is done and neurons can be exported.
Next: Branch types.

Note: Account for the removed branches due to loop merging.