Added particles pusher, grid and field class

-Add particle class
-Add particles class with method Push()
-Push method is relativistic Boris pusher
-Grid class is frame and grid where particle and field are
-Add Field class with two methods for getting E and B field, they are just primitive method for now, they are used just for testing at the moment
-Push method is tested and it works, boundary condition methos need to be implemeted if particles get outside of the frame
Merge branch 'master' of https://github.com/abukva/PyPIC