ENH: improvements to stringOps format and split functions

- split now optionally retains empty substrings.
  Added split on fixed field width.

- Foam::name() now formats directly into string buffer, which a
  removes one layer of copying and also avoids using a non-constexpr
  in the temporary.

STYLE: explicit type narrowing on zero-padded output for ensight

src/fileFormats/ensight/file/ensightCaseOptions.C

@@ -59,7 +59,7 @@ Foam::word Foam::ensightCase::options::padded(const label i) const
     // As per Foam::name, but with fixed length
     char buf[32];
 
-    ::snprintf(buf, 32, printf_.c_str(), i);
+    ::snprintf(buf, 32, printf_.c_str(), static_cast<int>(i));
     buf[31] = 0;
 
     // no stripping required