Compression helper?

[jonkeane]

Following #93 where compression will be set to None if it is None, uncompressed or none

In the future we might consider making this a bit less funky: using None meaningfully here is a bit odd compared to it being unspecified elsewhere... but the compression argument in pyarrow doesn't take uncompressed it takes None, so we would need to do a whole getter | setter thing for compression where we store "uncompressed" for all of them but then when we get it for writing we swap to None. Not sure if it's worth all of that, so let's kick that decision down the road.