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/ChargeRegulator.cpp

@@ -164,7 +164,7 @@ namespace ATC {
         chargeRegulator_(chargeRegulator),
         lammpsInterface_(LammpsInterface::instance()),
         rC_(0), rCsq_(0),
-        targetValue_(NULL), 
+        targetValue_(nullptr), 
         targetPhi_(p.value), 
         surface_(p.faceset),
         atomGroupBit_(p.groupBit),