Add support for sizes arrays (#64)

This PR adds the ability in the C++ library to read "sizes" arrays instead of the "size N index" arrays.  (The latter was missing the final element which had to be fixed up later in the code.)
* New function `sizes_to_indices` that converts a vector of sizes to a vector of indices
* Removed fixup code from `qv` ivf queries
* Updates to setup.bash to use sizes arrays rather than index arrays
* Renamed the CLI programs `ivf_hack` to `ivf_flat` and `flat` to `flat_l2`

Author: lums658