Use std::stringstream instead of snprintf() for double->string conversion

`snprintf()` will use the current `LC_NUMERIC` locale
for converting a double to a string,
which will use a `,` instead of a `.` in some locales (e.g. de_DE).
`std::stringstream` allows setting the locale to `"C"` to always get a `.`.
This occurs only for that `stringstream` instance; no global is
altered.