More multi-threading

See merge request !37

Enable multi-threading of data delivery (decompress, copy-out, etc.) for read. Including the reads done in finalize.

Speed up range() by using correct type in accumulator. Noticeable when writing int8 or int16 files. Also remove a hand-optimization that didn't have much effect.

Make LocalFileLinux::xx_readv() honor the request for parallel delivery. But don't use the feature yet.

Update Readme.md removing the alpha statements.

See merge request !36

Miscellaneous Minor Modifications

See merge request !35

Building without Seismic Store

See merge request !34