`PauliBasis` is misleadingly named as it doesn't represent the operator basis of Pauli matrices+identity

[akirakyle]

As discussed in #35, the basis currently named PauliBasis is actually a hilbert space basis equivalent to a CompositeBasis consisting of just num_qubits worth of `SpinBasis(1//2)'.

[Krastanov]

A potential solution is to rename it, make an alias with the old name, slap a deprecation warning on the alias, and remove any mention of the alias from the documentation.