ENH: simplify SubStrings class/handling (remove template parameters)

- previous code used derived string iterators, but these would
  be largely ignored anyhow since the underlying std::sub_match
  str() method would just yields a std::string anyhow.

  The SubStrings::str(size_t) method wasn't used in any code, so now
  just use std::string iterators only.

  This change simplfies overall handling, since it removes an unneeded
  template dependency.

applications/test/HashTable1/Make/files

@@ -1,3 +1,3 @@
-Test-HashTable1.C
+Test-HashTable1.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-HashTable1

applications/test/string/Make/files

@@ -1,3 +1,3 @@
-Test-string.C
+Test-string.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-string

applications/test/string/Test-string.cxx

@@ -89,7 +89,7 @@ int main(int argc, char *argv[])
 
         inputType in1("move-construct-from");
 
-        Info<<"move construct from " << in1.length() << nl;
+        Info<<"move construct from " << in1.size() << nl;
 
         outputType out1(std::move(in1));
 
@@ -100,7 +100,7 @@ int main(int argc, char *argv[])
         out1 = "some-text-rubbish";
         out1.resize(10);
 
-        Info<<"move assign from " << in1.length() << nl;
+        Info<<"move assign from " << in1.size() << nl;
 
         out1 = std::move(in1);
 
@@ -329,7 +329,7 @@ int main(int argc, char *argv[])
 
     string s2(s.expand());
 
-    cout<< "output string with " << s2.length() << " characters\n";
+    cout<< "output string with " << s2.size() << " characters\n";
     cout<< "ostream<<  >" << s2 << "<\n";
     Info<< "Ostream<<  >" << s2 << "<\n";
     Info<< "hash:" << hex << string::hasher()(s2) << dec << endl;

applications/test/stringList/Make/files

@@ -1,3 +1,3 @@
-Test-stringList.C
+Test-stringList.cxx
 
 EXE = $(FOAM_USER_APPBIN)/Test-stringList

applications/test/stringSplit/Test-stringSplit.cxx

@@ -43,7 +43,7 @@ template<class StringType>
 void printSubStrings
 (
     const StringType& str,
-    const SubStrings<StringType>& split
+    const SubStrings& split
 )
 {
     Info<< "string {" << str.size() << " chars} = " << str << nl