Avoid division by zero at 0 Hz in DistributedCircuit

[Ttl]

Adds small offset to avoid dividing by zero if frequency is zero. DC S-parameters are extremely slightly off since adding the offset adds very small imaginary part when DC S-parameters should be real. It's better than nans though and fixing it properly would need more radical changes.

These changes are enough to avoid nans when both or only either R or G is zero. Just adding offset to Y is not enough when R is zero.

import skrf
from skrf.media import DistributedCircuit
freq = skrf.F(0,0,1)
m = DistributedCircuit(frequency = freq,
                          z0 = 50,
                          C = 93.5e-12,
                          L = 273e-9,
                          R = 0,
                          G = 0)
line = m.line(10, 'm', z0=m.Z0, embed=True)
print(line.s)

Before:

[[[nan+nanj nan+nanj]
  [nan+nanj nan+nanj]]]

After:

[[[-4.4408921e-16-2.499e-10j  1.0000000e+00-2.501e-10j]
  [ 1.0000000e+00-2.501e-10j -4.4408921e-16-2.499e-10j]]]