fix key error in unbalanced powerflow if trafo index is not consecutive or not starting at zero

[jkupka]

Running pp.runpp_3ph fails with a key error if trafo index is not starting at 0 or it is not consecutive.

Minimal example:

import pandapower as pp
import pandapower.networks as nw

net = nw.ieee_european_lv_asymmetric("on_peak_566")
net.trafo.loc[1] = net.trafo.loc[0]
net.trafo.drop(0, inplace=True)

pp.runpp_3ph(net) # -> KeyError: 0

Fixed by this PR.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 79.65%. Comparing base (685ca90) to head (a0cf82e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2217   +/-   ##
========================================
  Coverage    79.65%   79.65%           
========================================
  Files          255      255           
  Lines        27841    27841           
========================================
  Hits         22178    22178           
  Misses        5663     5663           

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