Specific Population Functions

[thejmazz]

Related to #26.

Basically, "Pick and check for collisions at random positions to place proteins on membranes" can work well, but there needs to be a way to handle more specific structures. For example, ATP synthases on cristae. The dimer's need to pinch the membrane, and essentially be centering over the main pinching region. I attempted this using the collision based alg., plus also only allowing it to use vertices based on a z threshold, to "push" them close to the front. What is awesome is that this actually kinda works, and gets a "natural" dimer arrangement, but the threshold is too individually specific, and it won't work on all cristaes with the same value. You can see here one cristae is good (albeit a little offset from center), while another has too many ATP synthases: