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/improper.cpp

@@ -33,10 +33,10 @@ Improper::Improper(LAMMPS *lmp) : Pointers(lmp)
   suffix_flag = Suffix::NONE;
 
   maxeatom = maxvatom = maxcvatom = 0;
-  eatom = NULL;
-  vatom = NULL;
-  cvatom = NULL;
-  setflag = NULL;
+  eatom = nullptr;
+  vatom = nullptr;
+  cvatom = nullptr;
+  setflag = nullptr;
 
   execution_space = Host;
   datamask_read = ALL_MASK;