implement multipart download

[anwarnunez]

Something along the lines of: https://github.com/mumrah/s3-multipart

[anwarnunez]

bump!

[CMCDragonkai]

This would be good to benchmark it against local system access (especially memory mapped arrays). But this is really needed if you want to use chunking functionality or random-access into arrays rather than just sequential access where the arrays are larger than the memory of the computing program.