SWC types used in NeuroDevSim

The Front.swc_type refers to entries in a column of the SWC file format, which was defined in Cannon et al. 1998. NeuroDevSim uses standard values defined in this paper and on NeuroMorpho.org, but also adds several custom defined SWC types:

type	structure	comment
0	undefined	defined in Cannon et al.
1	soma	defined in Cannon et al.
2	axon	defined in Cannon et al.
3	(basal) dendrite	defined in Cannon et al.
4	apical dendrite	defined in Cannon et al.
5	custom	defined in Cannon et al.
6	neurite	defined by NeuroMorpho.org
7	glial process	defined by NeuroMorpho.org
8	oblique dendrite	NeuroDevSim definition
9	tuft dendrite	NeuroDevSim definition
10	smooth dendrite	NeuroDevSim definition
11	spiny dendrite	NeuroDevSim definition
12	filipodium	NeuroDevSim definition
13	spine	NeuroDevSim definition
14	synaptic bouton	NeuroDevSim definition
15-19	reserved	NeuroDevSim definition

Note that some of these definitions are required for NeuroDevSim to work properly. Somata automatically get swc_type=1, while some of Modeling soma migration depends on proper use of axon (2) and filipodium (12) swc_types.