examples/Particles/PIC2d/PIC2d.cpp works

[Roman Krylov]

It's about examples/Particles/PIC2d.
I made it compilable and runnable, though I have no experience in CVS, 
and even in RCS so I can't express it that way.
I had modified
    examples/Particles/PIC2d/PIC2d.cpp,
    src/Particles/Interpolation.cpp,
    src/Particles/InterpolatorNGP.h,
created 2D spec. of grad(vert2cell only) in src/Field/DiffOps/Grad.h and 
Grad.UR.h
In PIC2d.cpp I had to change '<some field>.x().comp(0)' to 
'iota(1,nx).comp(0)' for example to make it work and some other subtleties.
Does anybody working on it this time? Perhaps I'm wasting my time if 
it's all done already?
Roman.