Create the IV registry

[dalonsoa]

Creates the IV solver's registry and complete the tests for the registers, which were missing.

This should be reviewed and merged after #235

[codecov]

Codecov Report

Merging #239 (03b64dc) into develop (de18f91) will increase coverage by 1.16%.
The diff coverage is 79.31%.

@@             Coverage Diff             @@
##           develop     #239      +/-   ##
===========================================
+ Coverage    46.07%   47.23%   +1.16%     
===========================================
  Files           85       85              
  Lines         9133     9342     +209     
===========================================
+ Hits          4208     4413     +205     
- Misses        4925     4929       +4     

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	97.14% <100.00%> (-2.86%)	⬇️
solcore/absorption_calculator/tmm_core_vec.py	100.00% <0.00%> (ø)

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