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/fix.cpp

@@ -33,8 +33,8 @@ int Fix::instance_total = 0;
 
 Fix::Fix(LAMMPS *lmp, int /*narg*/, char **arg) :
   Pointers(lmp),
-  id(NULL), style(NULL), extlist(NULL), vector_atom(NULL), array_atom(NULL),
-  vector_local(NULL), array_local(NULL), eatom(NULL), vatom(NULL)
+  id(nullptr), style(nullptr), extlist(nullptr), vector_atom(nullptr), array_atom(nullptr),
+  vector_local(nullptr), array_local(nullptr), eatom(nullptr), vatom(nullptr)
 {
   instance_me = instance_total++;