[bug] Modifying the index of bus to a custom value results in an error

[258796535]

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Reproducible Example

net = pp.create_empty_network()

b1 = pp.create_bus(net, name="bus 1", vn_kv=1., index=2)
b2 = pp.create_bus(net, name="bus 2", vn_kv=1., index=3)
b3 = pp.create_bus(net, name="bus 3", vn_kv=1., index=4)

pp.create_ext_grid(net, b1)  # set the slack bus to bus 1

l1 = pp.create_line_from_parameters(net, 2, 3, 1, r_ohm_per_km=.01, x_ohm_per_km=.03,
                                    c_nf_per_km=0, max_i_ka=1)
l2 = pp.create_line_from_parameters(net, 2, 4, 1, r_ohm_per_km=.02, x_ohm_per_km=.05,
                                    c_nf_per_km=0, max_i_ka=1)
l3 = pp.create_line_from_parameters(net, 3, 4, 1, r_ohm_per_km=.03, x_ohm_per_km=.08,
                                    c_nf_per_km=0, max_i_ka=1)
pp.create_measurement(net, "v", "bus", 1.006, 0.004, b1) #母线1电压1.006  装置误差0.004
pp.create_measurement(net, "v", "bus", 0.968, 0.004, b2) #母线2电压0.968  装置误差0.004

#============母线2 的有功 、无功测量 与 误差========
pp.create_measurement(net, "p", "bus", -0.501, 0.01, b2)#母线2有功-0.501 标准偏差0.01
pp.create_measurement(net, "q", "bus", -0.266, 0.01, b2)#母线2无功 -.266  误差0.01

#===线路测量====
pp.create_measurement(net, "p", "line",0.888, 0.008, l1, side=1)# P_line (bus 1 -> bus 2) at bus 1 有功0.888, 误差80.008.
pp.create_measurement(net, "p", "line", 1.173,0.008, l2, side=1)#P_line (bus 1 -> bus 3) at bus 1
pp.create_measurement(net, "q", "line", 0.568, 0.008, l1, side=1)#q_line (bus 1 -> bus 2) at bus 1
pp.create_measurement(net, "q", "line", 0.663, 0.008, l2, side=1)#Q_line (bus 1 -> bus 3) at bus 1

#现在，我们可以执行状态估计

success = est.estimate(net, init="flat")
#V, delta = net.res_bus_est.vm_pu, net.res_bus_est.va_degree #
print("母线结果:\n",net.res_bus_est)
linesresult=pd.DataFrame(net.res_line_est)
linesresult.to_csv("线路结果.csv")
print("线路结果:\n",net.res_line_est)

Issue Description and Traceback

Measurements available: 4. Measurements required: 5
Traceback (most recent call last):
File "D:\workspace_py\energy-api\dvadmin\poweranalysis\test\state_estimation_example1.py", line 81, in
success = est.estimate(net, init="flat")
File "D:\workspace_py\energy-api\venv\lib\site-packages\pandapower\estimation\state_estimation.py", line 87, in estimate
return se.estimate(v_start=v_start, delta_start=delta_start,
File "D:\workspace_py\energy-api\venv\lib\site-packages\pandapower\estimation\state_estimation.py", line 267, in estimate
self.eppci = self.solver.estimate(self.eppci, **opt_vars)
File "D:\workspace_py\energy-api\venv\lib\site-packages\pandapower\estimation\algorithm\base.py", line 80, in estimate
self.initialize(eppci)
File "D:\workspace_py\energy-api\venv\lib\site-packages\pandapower\estimation\algorithm\base.py", line 61, in initialize
self.check_observability(eppci, eppci.z)
File "D:\workspace_py\energy-api\venv\lib\site-packages\pandapower\estimation\algorithm\base.py", line 44, in check_observability
raise UserWarning("Measurements available: %d. Measurements required: %d" %
UserWarning: Measurements available: 4. Measurements required: 5

Expected Behavior

if I use default value of example as shown below，the index value of bus start of 1 or 0，it is not error！
ps: I modify the line params about bus info,the net is right.
net = pp.create_empty_network()

b1 = pp.create_bus(net, name="bus 1", vn_kv=1., index=1)
b2 = pp.create_bus(net, name="bus 2", vn_kv=1., index=2)
b3 = pp.create_bus(net, name="bus 3", vn_kv=1., index=3)

pp.create_ext_grid(net, b1) # set the slack bus to bus 1

l1 = pp.create_line_from_parameters(net, 1, 2, 1, r_ohm_per_km=.01, x_ohm_per_km=.03,
c_nf_per_km=0, max_i_ka=1)
l2 = pp.create_line_from_parameters(net, 1, 3, 1, r_ohm_per_km=.02, x_ohm_per_km=.05,
c_nf_per_km=0, max_i_ka=1)
l3 = pp.create_line_from_parameters(net, 2, 3, 1, r_ohm_per_km=.03, x_ohm_per_km=.08,
c_nf_per_km=0, max_i_ka=1)

Installed Versions

• python version: * pandas version: * networkx version: * scipy version: * numpy version: * packaging version: * tqdm version: * deepdiff version: * Operating System name/version:2.13.1 or 2.14.11

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