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/USER-MISC/compute_gyration_shape.cpp

@@ -30,7 +30,7 @@ using namespace LAMMPS_NS;
 /* ---------------------------------------------------------------------- */
 
 ComputeGyrationShape::ComputeGyrationShape(LAMMPS *lmp, int narg, char **arg) :
-  Compute(lmp, narg, arg), id_gyration(NULL)
+  Compute(lmp, narg, arg), id_gyration(nullptr)
 {
   if (narg != 4) error->all(FLERR,"Illegal compute gyration/shape command");