[bug] merge_nets doesn't merge characteristic curves

[vogt31337]

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• Browse the tutorials and tests for usefull code snippets and examples of use

• Reproduced the issue after updating with pip install --upgrade pandapower (or git pull)

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Reproducible Example

import pandapower as pp
from pandapower.networks.simple_pandapower_test_networks import simple_four_bus_system
from pandapower.control.util.characteristic import Characteristic

# create two networks
net1 = simple_four_bus_system()
net2 = simple_four_bus_system()

# create two characteristic
Characteristic(net1, x_values=[0.85, 1.15], y_values=[5, 15])
Characteristic(net2, x_values=[0.95, 1.05], y_values=[10, 20])

# assign the characteristic ids to the trafos
net1.trafo.loc[:, "vk_percent_characteristic"] = 0
net2.trafo.loc[:, "vk_percent_characteristic"] = 0

# merge networks
merged = pp.merge_nets(net1, net2, validate=False)

# fail, since the second transformer should have the second characteristic
assert merged.trafo.loc[1, "vk_percent_characteristic"] == 1

Issue Description and Traceback

merge_nets doesn't update of characteristic in the fused network which leads to a problem in the pf.

Expected Behavior

correct merging of these characteristic

Installed Versions

INSTALLED VERSIONS

commit : 0f437949513225922d851e9581723d82120684a6
python : 3.11.4.final.0
python-bits : 64
OS : Windows
OS-release : 10
Version : 10.0.19045
machine : AMD64
processor : Intel64 Family 6 Model 142 Stepping 12, GenuineIntel
byteorder : little
LC_ALL : None
LANG : None
LOCALE : de_DE.cp1252
pandas : 2.0.3
numpy : 1.24.4
pytz : 2023.3
dateutil : 2.8.2
setuptools : 68.0.0
pip : 23.2.1
Cython : 3.0.0
pytest : 8.1.1
hypothesis : None
sphinx : None
blosc : None
feather : None
xlsxwriter : 3.1.2
lxml.etree : None
html5lib : None
pymysql : None
psycopg2 : 2.9.9
jinja2 : 3.1.2
IPython : 8.15.0
pandas_datareader: None
bs4 : 4.12.2
bottleneck : None
brotli : 1.0.9
fastparquet : None
fsspec : None
gcsfs : None
matplotlib : 3.7.1
numba : 0.57.1
numexpr : None
odfpy : None
openpyxl : 3.1.2
pandas_gbq : None
pyarrow : 16.0.0
pyreadstat : None
pyxlsb : None
s3fs : None
scipy : 1.11.1
snappy : None
sqlalchemy : None
tables : None
tabulate : None
xarray : None
xlrd : None
zstandard : None
tzdata : 2023.3
qtpy : 2.4.0
pyqt5 : None

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