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/USER-MISC/compute_viscosity_cos.cpp

@@ -45,7 +45,7 @@ ComputeViscosityCos::ComputeViscosityCos(LAMMPS *lmp, int narg, char **arg) :
   tempbias = 1;
 
   maxbias = 0;
-  vbiasall = NULL;
+  vbiasall = nullptr;
 
   vector = new double[7];
 }