Added cell-cell short range function.
Starting to build second level cells
Fixed the short range error.

I was not using the updated xyz values when calculating particle force errors
Moved new short range calc into tools.

Updated pic3d3v to call it correctly
Removed atmoics that were not required.
Collapsed loop
Moved outer short range loop to cpp
Cleaned up the short range calc call chain.
Moved omp over cell loop
Removed periodic check in cell interior E calc.

The interior cell calc does not worry about periodic neighbors by definition.
Removed periodic check from cell-cell force calcs.

Put in the loop over cell neighboors where it should be.
Fixed minor error in periodic calc.

The additive terms s has the incorrect sign.
In the dev notebook I was not generating random numbers on the whole domain, just on 0-1.
Logic to choose N_cells based on rmax and L
Added back support for S2 screens.

Put asserts in solver LR and calc_E_short_range to check for invalid screens.
Merge branch 'sr-opt' into p3m

Conflicts:
	py3d3v/pic3d3v.py
Remove short range calc cells.

Following a particle is complicated by the fact that we are changing their relative order for the short range cell based force calcs.