Grid3D: can't specify grid dimensions in constructor

[austin-hoover]

Grid1D and Grid2D can be constructed with either the grid shape:

PyORBIT3/src/spacecharge/Grid2D.cc

Line 12 in 709a5cc

Grid2D::Grid2D(int xSize, int ySize): CppPyWrapper(NULL)

or the grid shape and size along each dimension:

PyORBIT3/src/spacecharge/Grid2D.cc

Lines 24 to 26 in 709a5cc

	Grid2D::Grid2D(int xSize, int ySize,
	double xMin, double xMax,
	double yMin, double yMax): CppPyWrapper(NULL)

However Grid3D has only one constructor:

PyORBIT3/src/spacecharge/Grid3D.cc

Line 27 in 709a5cc

Grid3D::Grid3D(int nX, int nY, int nZ): CppPyWrapper(NULL)

Is there any reason not to add another constructor for Grid3D?