fix minor errors in stanet converter

[jkisse]

• stanet_nr is now -999 instead of NA (if applicable)
• remove xfails and unused imports
• fail-safe if no layers available
• fail-safe if slider diameter = 0

todo:

• add note in documentation (simulation in stanet required prior to export, HA pipes)

closes #390

[codecov]

Codecov Report

Base: 84.96% // Head: 85.14% // Increases project coverage by +0.18% 🎉

Coverage data is based on head (03adb94) compared to base (7c665e8).
Patch coverage: 85.71% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #461      +/-   ##
===========================================
+ Coverage    84.96%   85.14%   +0.18%     
===========================================
  Files           90       90              
  Lines         6072     6073       +1     
===========================================
+ Hits          5159     5171      +12     
+ Misses         913      902      -11     

Impacted Files	Coverage Δ
pandapipes/converter/stanet/stanet2pandapipes.py	77.92% <75.00%> (+0.70%)	⬆️
pandapipes/converter/stanet/table_creation.py	66.22% <100.00%> (+1.72%)	⬆️
pandapipes/converter/stanet/preparing_steps.py	81.30% <0.00%> (+0.86%)	⬆️

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[dlohmeier]

Nice, thanks for adding the tests and the quick fixes.