Add doubly linked list.
Add pure C implementation of GLM.
Add C++ compatibility to the C GLM library.
Fix concatenation bug.
Add octree implementation.
Fix bug in _octreeNodeRemove.
Comment out incomplete functions to avoid warnings.
Remove const qualifiers on output of functions.
Add treedemo target and file.

The program now requires gcc 4.6 or higher for C11 anonymous struct and union's.
Merge branch 'slowtree'

Premature merge of the slowtree branch in order to add the C GLM library and the
octree structure to the main branch.