Consistent use of string methods

• use more standard forms of string access. Eg, substr() instead of the operator()(...) form, or even resize().
• turn off underscore prefixing by default for word::validated()
• new SubStrings container and associated stringOps::split operator. This container may also be useful for writing simple parsers.

applications/test/string/Test-string.C

@@ -83,7 +83,8 @@ int main(int argc, char *argv[])
     Info<<"camel-case => " << (word("camel") & "case") << nl;
     for (const auto& s : { " text with \"spaces'", "08/15 value" })
     {
-        Info<<"validated \"" << s << "\" => " <<  word::validated(s) << nl;
+        Info<<"validated \"" << s << "\" => "
+            << word::validated(s, true) << nl;
     }
     Info<< nl;