Checksum should be disabled when compressing into memory

[FrancescAlted]

Checksum is a nice thing to have when you are writing to disk, but its use is more questionable for the in-memory API (e.g. pack_ndarray_str). I propose to disable the checksum whenever bloscpack is using memory as a backend to store results.

[esc]

I agree that it doesn't make sense in this scenario. However I am -0 about having inconsistent defaults. Though I am pondering changing the defaults to have no checksum, no offsets and thus also no append chunks, but I am not settled on this yet.

[FrancescAlted]

Well, if it helps, HDF5 includes support for checksumming, but it is disable by default, so +1 for disabling it by default here as well.

Regarding omitting the offsets by default, I would say that it is a good idea too. I plan to use bloscpack in bcolz in the near future, and appending is a critical feature there, but that can be activated programmatically as well, so +1 to remove offsets by default too.