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_slice.cpp

@@ -34,7 +34,7 @@ enum{COMPUTE,FIX,VARIABLE};
 
 ComputeSlice::ComputeSlice(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  nvalues(0), which(NULL), argindex(NULL), value2index(NULL), ids(NULL)
+  nvalues(0), which(nullptr), argindex(nullptr), value2index(nullptr), ids(nullptr)
 {
   if (narg < 7) error->all(FLERR,"Illegal compute slice command");