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_rdf.cpp

@@ -41,9 +41,9 @@ using namespace MathConst;
 
 ComputeRDF::ComputeRDF(LAMMPS *lmp, int narg, char **arg) :
   Compute(lmp, narg, arg),
-  rdfpair(NULL), nrdfpair(NULL), ilo(NULL), ihi(NULL), jlo(NULL), jhi(NULL),
-  hist(NULL), histall(NULL), typecount(NULL), icount(NULL), jcount(NULL),
-  duplicates(NULL)
+  rdfpair(nullptr), nrdfpair(nullptr), ilo(nullptr), ihi(nullptr), jlo(nullptr), jhi(nullptr),
+  hist(nullptr), histall(nullptr), typecount(nullptr), icount(nullptr), jcount(nullptr),
+  duplicates(nullptr)
 {
   if (narg < 4) error->all(FLERR,"Illegal compute rdf command");