Switch to local Xtk and xtk_dat.
Add 'default' struct to hemi_select(...).
Improve into comments.

Add new fields to core (S_render) structure:

name
visible
colorInterpolation
Minor comments update.
Major refactoring. Add 'defaults' for lh/rh and related structures. Use 'defaults'
for initial (and possibly updated) field values. Handle URL-passed values.
Update to better handle use of lh/rh defaults -- used when subject
is refreshed/changed.
Add prototype subArray_set for Float32.

Detect color interpolation from S_render object (and hence URL
passed values).