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/lmppython.cpp

@@ -26,7 +26,7 @@ Python::Python(LAMMPS *lmp) : Pointers(lmp)
 {
   // implementation of Python interface is only loaded on demand
   // and only if PYTHON package has been installed and compiled into binary
-  impl = NULL;
+  impl = nullptr;
 }
 
 /* ---------------------------------------------------------------------- */