The current error handler for the `PwBaseWorkChain` that deals with SCF convergence
issues only tries to reduce the `mixing-beta` by 20% each time the calculation fails,
and doesn't consider the rate of convergence.

Here we improve the error handling for this case by calculating the slope on the
logarithm of the "scf accuracy". Based on some statistics presented in

aiidateam#961

We noted two points:

1. Most of the calculations that converge do so within 50 SCF steps.
2. For those that don't, there is a very low chance of convergence in case the scf
   accuracy slope is higher than -0.1.

Hence, we:

* Limit the default number of maximum steps (`electron_maxstep`) to 50.
* In case of SCF convergence failure, check the scf accuracy slope. If < -0.1, do a
  full restart.
* Else, check the `mixing_beta`. If above 0.1, half it and restart the calculation.

* Stick to 50 `electron_maxstep`
* Refuse to increase number of SCF step if slope good
* Switch to `local-TF` mixing if lowsym or lowdim
* Finally, reduce mixing beta and increase ndim