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_minimize.cpp

@@ -23,7 +23,7 @@ using namespace FixConst;
 
 FixMinimize::FixMinimize(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  nvector(0), peratom(NULL), vectors(NULL)
+  nvector(0), peratom(nullptr), vectors(nullptr)
 {
   // register callback to this fix from Atom class
   // don't perform initial allocation here, must wait until add_vector()