v0.4 summary issue

[rtimms]

Roadmap issues:

• [x] blended electrodes https://github.com/pybamm-team/BPX/issues/23
• [x] validation based on models https://github.com/pybamm-team/BPX/issues/29
• [x] "additional" parameters https://github.com/pybamm-team/BPX/issues/26
• [ ] split validity from completeness https://github.com/pybamm-team/BPX/issues/40
• [x] extra examples to show off hysteresis (via additional parameters) and blended electrodes
• [x] API docs
• [x] user guide
• [x] change a_n and a_p to b_n and b_p (the surface area per unit volume) in eq. (6c) in the BPX document -- a small bug fix

Extras:

• [ ] active material volume specification https://github.com/pybamm-team/BPX/issues/41

[ejfdickinson]

As discussed earlier, I agree with the suggestion to delay #40 to a future release.