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/body.cpp

@@ -23,8 +23,8 @@ Body::Body(LAMMPS *lmp, int /*narg*/, char **arg) : Pointers(lmp)
   int n = strlen(arg[0]) + 1;
   style = new char[n];
   strcpy(style,arg[0]);
-  icp = NULL;
-  dcp = NULL;
+  icp = nullptr;
+  dcp = nullptr;
 }
 
 /* ---------------------------------------------------------------------- */