Feature/integrate helm solver

[vogt31337]

Hi,

I've created a PR which enables the use of the HELM method for pandapower. For this to function HELMPy needs to be installed. At the moment a modified Version must be used (this one here HELMPy), this version returns more values, for example if the method did converge and how many pade approximations needed to be done to reach the target accuracy.

BR

[codecov]

Codecov Report

Attention: Patch coverage is 22.34043% with 73 lines in your changes missing coverage. Please review.

Project coverage is 78.66%. Comparing base (70071d2) to head (580546d).
Report is 863 commits behind head on develop.

Files	Patch %	Lines
pandapower/pf/run_helmpy_pf.py	17.64%	70 Missing ⚠️
pandapower/powerflow.py	33.33%	2 Missing ⚠️
pandapower/auxiliary.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2067      +/-   ##
===========================================
- Coverage    78.86%   78.66%   -0.21%     
===========================================
  Files          213      214       +1     
  Lines        26541    26634      +93     
===========================================
+ Hits         20932    20952      +20     
- Misses        5609     5682      +73     

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[rbolgaryn]

I think there should be more tests for this implementation

[rbolgaryn]

Furthermore, documentation and CHANGELOG should be extended.

[rbolgaryn]

see comments above

[vogt31337]

still some bugs, for some networks results match, for others results don't match. I'm not sure why atm. Will investigate further.