add KSpace style zero to KSPACE and zero2 as plugin demo

examples/plugins/kspace_zero2.h

@@ -0,0 +1,33 @@
+/* -*- c++ -*- ----------------------------------------------------------
+   LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
+   https://www.lammps.org/ Sandia National Laboratories
+   LAMMPS development team: developers@lammps.org
+
+   Copyright (2003) Sandia Corporation.  Under the terms of Contract
+   DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
+   certain rights in this software.  This software is distributed under
+   the GNU General Public License.
+
+   See the README file in the top-level LAMMPS directory.
+------------------------------------------------------------------------- */
+
+#ifndef LMP_KSPACE_ZERO2_H
+#define LMP_KSPACE_ZERO2_H
+
+#include "kspace.h"
+
+namespace LAMMPS_NS {
+
+class KSpaceZero2 : public KSpace {
+ public:
+  KSpaceZero2(class LAMMPS *);
+  ~KSpaceZero2() override;
+
+  void init() override;
+  void setup() override;
+  void settings(int, char **) override;
+
+  void compute(int, int) override;
+};
+}    // namespace LAMMPS_NS
+#endif