add_cpg_counts does not work

`wgbstools add_cpg_counts  --verbose <filename.bam> `

stops with

`FileNotFoundError: [Errno 2] No such file or directory: './<filename>_chr15.output.bam'`


(`filename.bam` only contains reads mapping on chr15).


It also creates an indexed output file `<filename.counts.bam>`
which is empty

```
samtools view <filename.counts.bam> | wc -l
0
```

