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/atom_vec.cpp

@@ -2060,7 +2060,7 @@ void AtomVec::write_vel(FILE *fp, int n, double **buf)
 }
 
 /* ----------------------------------------------------------------------
-   pack bond info for data file into buf if non-NULL
+   pack bond info for data file into buf if non-nullptr
    return count of bonds from this proc
    do not count/pack bonds with bondtype = 0
    if bondtype is negative, flip back to positive
@@ -2118,7 +2118,7 @@ void AtomVec::write_bond(FILE *fp, int n, tagint **buf, int index)
 }
 
 /* ----------------------------------------------------------------------
-   pack angle info for data file into buf if non-NULL
+   pack angle info for data file into buf if non-nullptr
    return count of angles from this proc
    do not count/pack angles with angletype = 0
    if angletype is negative, flip back to positive