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_dipole_chunk.cpp

@@ -34,9 +34,9 @@ enum { MASSCENTER, GEOMCENTER };
 
 ComputeDipoleChunk::ComputeDipoleChunk(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  idchunk(NULL), massproc(NULL), masstotal(NULL), chrgproc(NULL),
-  chrgtotal(NULL), com(NULL),
-  comall(NULL), dipole(NULL), dipoleall(NULL)
+  idchunk(nullptr), massproc(nullptr), masstotal(nullptr), chrgproc(nullptr),
+  chrgtotal(nullptr), com(nullptr),
+  comall(nullptr), dipole(nullptr), dipoleall(nullptr)
 {
   if ((narg != 4) && (narg != 5))
     error->all(FLERR,"Illegal compute dipole/chunk command");