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/compute_chunk_atom.h

@@ -96,7 +96,7 @@ class ComputeChunkAtom : public Compute {
   class FixStore *fixstore;
 
   class Fix *lockfix;         // ptr to FixAveChunk that is locking out setups
-                              // NULL if no lock currently in place
+                              // nullptr if no lock currently in place
   bigint lockstart,lockstop;  // timesteps for start and stop of locking
 
   bigint invoked_setup;    // last timestep setup_chunks and nchunk calculated