Make netlist drawing more robust

[schlatterbeck]

The following little script consume very much memory very fast due to a broken netlist. The netlist is parsed successfully but the attempt to display it seems to run into an infinite recursion. (The netlist resulted from a broken qucsstudio filter designer output combined with a broken parser for the qucsstudio format I'm currently writing: Not something that anyone sane would create by hand but maybe a good example for a regression test)

>>> from lcapy import show_version
>>> show_version()
Python: 3.11.2 (main, Mar 13 2023, 12:18:29) [GCC 12.2.0]
SymPy: 1.11.1
NumPy: 1.24.2
Matplotlib: 3.6.3
SciPy: 1.10.1
Lcapy: 1.17

#!/usr/bin/python3

from lcapy import Circuit

c = Circuit ("""
P1 1 0_0 ; down
L1 1 2 ; right
C1 1_0 3 ; right
C2 3 0_1 ; down
L2 3 0_2 ; down
L3 3 4 ; right
C3 3 5 ; right
P_lowFreq 5 0_3 ; down
P_highFreq 5 0_4 ; down
W 0_0 0_1 ; right
W 0_1 0_2 ; right
W 0_2 0_3 ; right
W 0_3 0_4 ; right
""")

c.draw ()

[mph-]

Ha! What a nasty netlist! it's not topologically possible. This patch should throw an exception now.