Add SISO WER to high-level Python api

[thequilo]

Closes #110.

Add the SISO WER as meeteval.wer.sisower to the high-level Python api.

I named it sisower instead of wer to avoid conflicts with the module meeteval.wer.wer. I didn't change the CLI command name, it is still just wer.

The new API function now also supports STM and SegLST files when the file names are unique. Previously, only keyed text files were supported. This was done to avoid accidentally calling the SISO WER when you wanted to compute, e.g., the cpwer. But I now think that it is very unlikely to have unique filenames / session_ids when you don't want to compute the SISO WER.