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_spring_rg.cpp

@@ -67,7 +67,7 @@ void FixSpringRG::init()
 {
   masstotal = group->mass(igroup);
 
-  // if rg0 was specified as NULL, compute current Rg
+  // if rg0 was specified as nullptr, compute current Rg
   // only occurs on 1st run
 
   if (rg0_flag) {