adde file for particle class
added wall collision functions
added function stumps for collision handling and imports. Also added the
forgotten mass of the particle.
Bugfix: Messed up x and y walls
added bouncing off walls
bouncing of other particles added
bugfixes in collission time calculation. Also added simple test case in
main.
added advance fuction to move particle without collission for time t.
bugfix: Number of collissions is updated after collission resolution
now.
bugfixes

bugfixes in collission and simple testing
Merge remote-tracking branch 'origin/particle'