Fix a bug where creating a sid bcolz file when the containing directory
was already occupied by a sid caused an OSError on attempt of creating
the directory because it already existed.
e.g. if there were two sids, `1` and `2`. The paths would be
`00/00/000001.bcolz` and `00/00/000002.bcolz` which share the same
directory `00/00`.
Fixed by checking for directory existence before calling `makedirs`.
Add test coverage which exercises writing of sids that are siblings in
the sid directory structure.
Eddie Hebert
984e934e83