periodic boundary conditions - triclinic

Triclinic boundary conditions transormation of particle x,y,z coordinates to u1,u2,u3 coordinate space represented by non-orthognal box vectors. Ghost particles at less than half box distances away can then be identified and handled.

Not sure if coordinate space is required to be reverted back to x,y,z for all functions to work, but would probably be a good idea to decrease confusion for any other possible added functions or attribute outputs.