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_viscous.cpp

@@ -27,7 +27,7 @@ using namespace FixConst;
 
 FixViscous::FixViscous(LAMMPS *lmp, int narg, char **arg) :
   Fix(lmp, narg, arg),
-  gamma(NULL)
+  gamma(nullptr)
 {
   dynamic_group_allow = 1;