add support for command plugins with example

examples/plugins/helloplugin.cpp

@@ -0,0 +1,49 @@
+
+#include "lammpsplugin.h"
+
+#include "comm.h"
+#include "error.h"
+#include "pointers.h"
+#include "version.h"
+
+#include <cstring>
+
+namespace LAMMPS_NS {
+  class Hello : protected Pointers {
+    public:
+      Hello(class LAMMPS *lmp) : Pointers(lmp) {};
+      void command(int, char **);
+  };
+}
+
+using namespace LAMMPS_NS;
+
+void Hello::command(int argc, char **argv)
+{
+   if (argc != 1) error->all(FLERR,"Illegal hello command");
+   if (comm->me == 0)
+     utils::logmesg(lmp,fmt::format("Hello, {}!\n",argv[0]));
+}
+
+static void hellocreator(LAMMPS *lmp, int argc, char **argv)
+{
+    Hello hello(lmp);
+    hello.command(argc,argv);
+}
+
+extern "C" void lammpsplugin_init(void *lmp, void *handle, void *regfunc)
+{
+  lammpsplugin_t plugin;
+  lammpsplugin_regfunc register_plugin = (lammpsplugin_regfunc) regfunc;
+
+  plugin.version = LAMMPS_VERSION;
+  plugin.style   = "command";
+  plugin.name    = "hello";
+  plugin.info    = "Hello world command v1.0";
+  plugin.author  = "Axel Kohlmeyer (akohlmey@gmail.com)";
+  plugin.creator1 = nullptr;
+  plugin.creator2 = nullptr;
+  plugin.creator3 = (lammpsplugin_factory3 *) &hellocreator;
+  plugin.handle  = handle;
+  (*register_plugin)(&plugin,lmp);
+}