WIP: Add base representation of SparseObservable

This adds the very base representation of `SparseObservable`, including
the lowest-level constructor from Python space and the ability to view
the data buffers.  It does not add any of the means to calculate with
it, nor any of the high-level interfaces to construction.

This WIP commit will be amended to at least add documentation and tests
of the contents.

Author: jakelishman

crates/accelerate/src/lib.rs

@@ -26,6 +26,7 @@ pub mod pauli_exp_val;
 pub mod results;
 pub mod sabre;
 pub mod sampled_exp_val;
+pub mod sparse_observable;
 pub mod sparse_pauli_op;
 pub mod stochastic_swap;
 pub mod synthesis;