Test Carsons with the full P/Q expansion

[AnjoMan]

Currently, the Carsons model always uses a truncated expansion for computing the P/Q terms at each i/j combination:

carsons/carsons/carsons.py

Line 139 in d58461d

def compute_Q(self, i, j, number_of_terms=2):

Ideally we would expose the number of terms so a user can configure them, and we would have a reliable example calculation to use as a test case where, if you don't use the full P/Q series, the test would fail because the impedance is not calculated precisely enough.

One possible approach would be to expose a FullCarsons and ModifiedCarsons, which is how the Kersting source describes the simplification of taking only the first 1--2 terms of P/Q. The ModifiedCarsons model is needed for all cable calculations, since it can be formulated to not rely on having a y position. An overhead line can be supported by both the Full and Modified carsons equations