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

lib/atc/Stress.cpp

@@ -299,10 +299,10 @@ void StressCubicElasticDamped::stress(const FIELD_MATS      &fields,
 // cauchy born model
 //==============================================================================
 StressCauchyBorn::StressCauchyBorn(fstream &fileId, CbData &cb)
-  :  cblattice_(NULL),
-     potential_(NULL),
+  :  cblattice_(nullptr),
+     potential_(nullptr),
      makeLinear_(false),
-     cubicMat_(NULL),
+     cubicMat_(nullptr),
      initialized_(false),
      fixed_temperature_(0.),
      cbdata_(cb)