T2345: Replace instances of NULL with nullptr

The following changes have been applied to src and lib folders:
regex replace: ([^"_])NULL ⇒ \1nullptr (8968 chgs in src, 1153 in lib)
Manually find/change: (void \*) nullptr ⇒ nullptr (1 case)
regex find: ".*?nullptr.*?"
  Manually ~14 cases back to "NULL" in src, ~2 in lib
  regex finds a few false positive where nullptr appears between two
  strings in a function call

src/PYTHON/fix_python_move.cpp

@@ -37,7 +37,7 @@ FixPythonMove::FixPythonMove(LAMMPS *lmp, int narg, char **arg) :
 
   python->init();
 
-  py_move = NULL;
+  py_move = nullptr;
 
   PyGILState_STATE gstate = PyGILState_Ensure();
 
@@ -50,7 +50,7 @@ FixPythonMove::FixPythonMove(LAMMPS *lmp, int narg, char **arg) :
   char * full_cls_name = arg[3];
   char * lastpos = strrchr(full_cls_name, '.');
 
-  if (lastpos == NULL) {
+  if (lastpos == nullptr) {
     error->all(FLERR,"Fix python/integrate requires fully qualified class name");
   }
 
@@ -138,7 +138,7 @@ void FixPythonMove::init()
     PyGILState_Release(gstate);
     error->all(FLERR,"Could not find 'init' method'");
   }
-  PyObject * result = PyEval_CallObject(py_init, NULL);
+  PyObject * result = PyEval_CallObject(py_init, nullptr);
   PyGILState_Release(gstate);
 }
 
@@ -174,7 +174,7 @@ void FixPythonMove::final_integrate()
     PyGILState_Release(gstate);
     error->all(FLERR,"Could not find 'final_integrate' method'");
   }
-  PyObject * result = PyEval_CallObject(py_final_integrate, NULL);
+  PyObject * result = PyEval_CallObject(py_final_integrate, nullptr);
   PyGILState_Release(gstate);
 }
 
@@ -229,6 +229,6 @@ void FixPythonMove::reset_dt()
     PyGILState_Release(gstate);
     error->all(FLERR,"Could not find 'reset_dt' method'");
   }
-  PyObject * result = PyEval_CallObject(py_reset_dt, NULL);
+  PyObject * result = PyEval_CallObject(py_reset_dt, nullptr);
   PyGILState_Release(gstate);
 }