Describe the bug
I found an example where the ext grid is set out of service by the connectivity check during the hydraulic calculation. The problem: It seems that it is still used as slack in the heat transfer calculation. This can be shown by setting the ext_grid out of service beforehand and restarting the pipeflow. It raises a heat transfer calculation did not converge.

To Reproduce
Use the test case test_connectivity_heat4 in test_inservice.py. Set the ext_grid 2 out of service manually. The pipeflow for the heat transfer does not converge anymore.

Error message
File "C:\Users\sdrauz\git\pandapipes\pandapipes\pipeflow.py", line 111, in pipeflow
raise PipeflowNotConverged("The heat transfer calculation did not converge to a "
pandapipes.pipeflow.PipeflowNotConverged: The heat transfer calculation did not converge to a solution.

Expected behavior
In both use cases the results should be the same

Python environment (please complete the following information):

• OS: Windows 10
• pandapipes version: 0.8.5
• Package Version

alabaster 0.7.13
anyio 3.6.2
appdirs 1.4.4
argon2-cffi 21.3.0
argon2-cffi-bindings 21.2.0
arrow 1.2.3
asttokens 2.2.1
attrs 22.2.0
Babel 2.11.0
backcall 0.2.0
beautifulsoup4 4.11.2
bleach 6.0.0
branca 0.6.0
brotlipy 0.7.0
cached-property 1.5.2
certifi 2022.12.7
cffi 1.15.1
charset-normalizer 2.1.1
click 8.1.3
click-plugins 1.1.1
cligj 0.7.2
colorama 0.4.6
comm 0.1.2
contourpy 1.0.7
coverage 7.0.5
cryptography 39.0.0
cycler 0.11.0
Cython 0.29.33
debugpy 1.6.6
decorator 5.1.1
deepdiff 6.2.3
defusedxml 0.7.1
dnspython 2.3.0
docutils 0.19
et-xmlfile 1.0.1
exceptiongroup 1.1.0
executing 1.2.0
fastjsonschema 2.16.2
Fiona 1.8.22
folium 0.14.0
fonttools 4.38.0
fqdn 1.5.1
GDAL 3.6.2
geopandas 0.12.2
h5py 3.7.0
html5lib 1.1
idna 3.4
igraph 0.10.4
imagesize 1.4.1
importlib-metadata 6.0.0
iniconfig 2.0.0
ipykernel 6.20.2
ipython 8.9.0
ipython-genutils 0.2.0
ipywidgets 8.0.4
isodate 0.6.1
isoduration 20.11.0
jedi 0.18.2
Jinja2 3.1.2
joblib 1.2.0
jsonpickle 1.5.1
jsonpointer 2.3
jsonschema 4.17.3
jupyter 1.0.0
jupyter_client 8.0.2
jupyter-console 6.4.4
jupyter_core 5.2.0
jupyter-events 0.6.3
jupyter_server 2.2.0
jupyter_server_terminals 0.4.4
jupyterlab-pygments 0.2.2
jupyterlab-widgets 3.0.5
kiwisolver 1.4.4
latexcodec 2.0.1
llvmlite 0.39.1
mapclassify 2.5.0
MarkupSafe 2.1.2
marshmallow 3.19.0
matplotlib 3.6.3
matplotlib-inline 0.1.6
mistune 2.0.4
munch 2.5.0
munkres 1.1.4
nbclassic 0.5.1
nbclient 0.7.2
nbconvert 7.2.9
nbformat 5.7.3
nest-asyncio 1.5.6
networkx 3.0
notebook 6.5.2
notebook_shim 0.2.2
numba 0.56.4
numexpr 2.7.3
numpy 1.23.5
numpydoc 1.5.0
openpyxl 3.0.10
ordered-set 4.1.0
orjson 3.8.5
packaging 23.0
pandas 1.5.3
pandocfilters 1.5.0
parso 0.8.3
patsy 0.5.3
pickleshare 0.7.5
Pillow 9.4.0
pip 22.3.1
platformdirs 2.6.2
pluggy 1.0.0
ply 3.11
pooch 1.6.0
prometheus-client 0.16.0
prompt-toolkit 3.0.36
psutil 5.9.4
pure-eval 0.2.2
pybtex 0.24.0
pybtex-docutils 1.0.2
pycparser 2.21
Pygments 2.14.0
pymongo 4.3.3
Pyomo 6.4.4
pyOpenSSL 23.0.0
pyparsing 3.0.9
pyproj 3.4.1
pyrsistent 0.19.3
PySocks 1.7.1
pytest 7.2.1
python-dateutil 2.8.2
python-igraph 0.10.4
python-json-logger 2.0.4
pytz 2022.7.1
pywin32 305
pywinpty 2.0.10
PyYAML 6.0
pyzmq 25.0.0
qtconsole 5.4.0
QtPy 2.3.0
rdflib 6.2.0
requests 2.28.2
rfc3339-validator 0.1.4
rfc3986-validator 0.1.1
Rtree 1.0.1
scikit-learn 1.2.0
scipy 1.10.0
seaborn 0.12.2
Send2Trash 1.8.0
setuptools 66.1.1
shapely 2.0.0
simbench 1.3.0
six 1.16.0
sniffio 1.3.0
snowballstemmer 2.2.0
soupsieve 2.3.2.post1
Sphinx 6.1.3
sphinx-rtd-theme 0.5.2
sphinxcontrib-applehelp 1.0.4
sphinxcontrib-bibtex 2.5.0
sphinxcontrib-devhelp 1.0.2
sphinxcontrib-htmlhelp 2.0.0
sphinxcontrib-jsmath 1.0.1
sphinxcontrib-qthelp 1.0.3
sphinxcontrib-serializinghtml 1.1.5
stack-data 0.6.2
statsmodels 0.13.5
tables 3.7.0
terminado 0.17.1
texttable 1.6.7
threadpoolctl 3.1.0
tinycss2 1.2.1
tomli 2.0.1
tornado 6.2
tqdm 4.64.1
traitlets 5.9.0
typing_extensions 4.4.0
unicodedata2 15.0.0
uri-template 1.2.0
urllib3 1.26.14
wcwidth 0.2.6
webcolors 1.12
webencodings 0.5.1
websocket-client 1.5.0
wheel 0.38.4
widgetsnbextension 4.0.5
win-inet-pton 1.1.0
xlrd 2.0.1
XlsxWriter 3.0.7
xyzservices 2022.9.0
zipp 3.11.0