Notebooks and tutorials

[joshcombes]

make all notebooks more pedagogical

• [x] state and process tomography (https://github.com/rigetti/forest-benchmarking/pull/155)
• [x] entangled states (https://github.com/rigetti/forest-benchmarking/pull/149)
• [x] readout (https://github.com/rigetti/forest-benchmarking/pull/156)
• [x] Randomized Benchmarking + variants (https://github.com/rigetti/forest-benchmarking/pull/154)
• [ ] Phase estimation (do this last?)
• [x] Qubit spec (T1, T2, Rabi, Ramsey) (https://github.com/rigetti/forest-benchmarking/pull/175)
• [x] Direct fidelity estimation (https://github.com/rigetti/forest-benchmarking/pull/165)
• [x] quantum volume (https://github.com/rigetti/forest-benchmarking/pull/167)
• [x] random operators (https://github.com/rigetti/forest-benchmarking/pull/140)
• [x] superoperators (https://github.com/rigetti/forest-benchmarking/pull/140)
• [x] adder (https://github.com/rigetti/forest-benchmarking/pull/148)
• [x] distance measures (https://github.com/rigetti/forest-benchmarking/pull/153)