supertensor: Expose index and block offsets to python

I ended up not using these, but might as well check them in anyways.
high_board_t: Remove needless turn argument
Abstract supertensor code over a generic data interface

read_file_t and write_file_t representing anything that supports
pread and pwrite, respectively.  Currently only versions for local
files exist, but cloud support is now easy to add.
tensor-info: Update
Test reading slices from Rackspace