Switch from Vec2 to glm::vec2
Parameterize init()

Parameterize init() and remove simulation "roof" to allow for large
particle counts.
Increase simulation stability

Adjust sigma/beta parameters and move Verlet integration down to increase
simulation stability.  Otherwise at 2048 particles the simulation would blow up.

Parameters taken from this previous version:
http://web.archive.org/web/20090608033344/http://blog.brandonpelfrey.com/wp-content/tutorial_3.cpp
Add benchmark routine and results
Remove source index from neighbor struct

Remove the source index (::i) from the the neighbor struct since everywhere it
was used the current particles index could be used instead.
Const-ify unmodified temporaries
Cleanup