make sure `sort` uses specified temp directory, refs #52

[amirkdv]

See #52.

Additionally I suggest (not in this PR) changing the default value for TMP_DIR from os.getcwd() to OUTPUT_DIR/tmp because:

• the current working directory may not necessarily be on a disk that has lots of space, esp. in the docker usage where only the input and output directories are mounted to the container.
• the user is guaranteed to be aware that the output directory must have lots of space to hold the results.
• input directory could also be considered but it is common to mount input directories in read-only mode onto docker containers to avoid accidental file corruption.

[jaudoux]

Thanks again @amirkdv for this new PR. I also agree with you about the location of the TMP_DIR by default, I'll update that!