Grid3D: can only bin bunch along x-y-z axis

[austin-hoover]

The Grid2D::binBunch method bins particles on the x-y axis by default:

PyORBIT3/src/spacecharge/Grid2D.cc

Lines 113 to 115 in 709a5cc

	void Grid2D::binBunch(Bunch* bunch){
	this->binBunch(bunch,0,2);
	}

but also allows specifying the axis:

PyORBIT3/src/spacecharge/Grid2D.cc

Line 120 in 709a5cc

void Grid2D::binBunch(Bunch* bunch, int ind0, int ind1){

However, Grid3D::binBunch bins coordinates only on the x-y-z axis:

PyORBIT3/src/spacecharge/Grid3D.cc

Line 308 in 709a5cc

void Grid3D::binBunch(Bunch* bunch){