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/compute_centro_atom.cpp

@@ -36,7 +36,7 @@ using namespace LAMMPS_NS;
 
 ComputeCentroAtom::ComputeCentroAtom(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  distsq(NULL), nearest(NULL), centro(NULL)
+  distsq(nullptr), nearest(nullptr), centro(nullptr)
 {
   if (narg < 4 || narg > 6)
     error->all(FLERR,"Illegal compute centro/atom command");
@@ -88,7 +88,7 @@ ComputeCentroAtom::~ComputeCentroAtom()
 
 void ComputeCentroAtom::init()
 {
-  if (force->pair == NULL)
+  if (force->pair == nullptr)
     error->all(FLERR,"Compute centro/atom requires a pair style be defined");
 
   int count = 0;