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/random_mars.cpp

@@ -27,7 +27,7 @@ enum{ADD,SUBTRACT};
 /* ---------------------------------------------------------------------- */
 
 RanMars::RanMars(LAMMPS *lmp, int seed) : Pointers(lmp),
-  u(NULL)
+  u(nullptr)
 {
   int ij,kl,i,j,k,l,ii,jj,m;
   double s,t;