Add equilibrium solver registry and updates equilibrium_pdd

[dalonsoa]

Creates the equilibrium solver registry and updates the equilibrium_pdd function to use it. As a bonus, the ProcessStructure function has also been refactored, leaving the code a bit cleaner and better structured. Docstrings have been improved in both cases.

@phoebe-p , ignore the changes in the Jupyter notebook files - that has been a side effect of running the pre-commit hooks, in charge - among other things - of stripping notebooks of some metadata that changes every time they are open. Nothing to do with the actual changes described above.

[codecov]

Codecov Report

Merging #235 (39f7d0b) into develop (de18f91) will decrease coverage by 0.02%.
The diff coverage is 76.00%.

@@             Coverage Diff             @@
##           develop     #235      +/-   ##
===========================================
- Coverage    46.07%   46.05%   -0.03%     
===========================================
  Files           85       85              
  Lines         9133     9135       +2     
===========================================
- Hits          4208     4207       -1     
- Misses        4925     4928       +3     

Impacted Files	Coverage Δ
solcore/solar_cell_solver.py	69.51% <14.28%> (-1.30%)	⬇️
...poisson_drift_diffusion/DriftDiffusionUtilities.py	85.88% <100.00%> (-0.28%)	⬇️
solcore/registries.py	100.00% <100.00%> (ø)

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