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_gyration_chunk.cpp

@@ -29,8 +29,8 @@ using namespace LAMMPS_NS;
 
 ComputeGyrationChunk::ComputeGyrationChunk(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  idchunk(NULL), massproc(NULL), masstotal(NULL), com(NULL), comall(NULL),
-  rg(NULL), rgall(NULL), rgt(NULL), rgtall(NULL)
+  idchunk(nullptr), massproc(nullptr), masstotal(nullptr), com(nullptr), comall(nullptr),
+  rg(nullptr), rgall(nullptr), rgt(nullptr), rgtall(nullptr)
 {
   if (narg < 4) error->all(FLERR,"Illegal compute gyration/chunk command");