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_pair.cpp

@@ -28,7 +28,7 @@ enum{EPAIR,EVDWL,ECOUL};
 
 ComputePair::ComputePair(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  pstyle(NULL), pair(NULL), one(NULL)
+  pstyle(nullptr), pair(nullptr), one(nullptr)
 {
   if (narg < 4) error->all(FLERR,"Illegal compute pair command");
 
@@ -82,7 +82,7 @@ ComputePair::ComputePair(LAMMPS *lmp, int narg, char **arg) :
     extvector = 1;
     one = new double[npair];
     vector = new double[npair];
-  } else one = vector = NULL;
+  } else one = vector = nullptr;
 }
 
 /* ---------------------------------------------------------------------- */