mmcif writer: move stream handling to StarWriter::Write

Before, we already opened a file and the respective stream at
Writer initialization. This is weird when Write never gets called and
and empty file remains on disk. Furthermore, Write just wrote to a stream.
That stream was never flushed, so in principle you potentially called
Write but even after completion, the file is not necessarily written
to disk. Now, the file gets opened in Write, everything is written
to the respective stream and once done, the destructor of the stream
ensures that everything concludes.