Export generalized (N×N) capacitance and inductance matrices

[Copilot]

Currently pulmtln.out.json only exports the reduced (N-1)×(N-1) C and L matrices. This adds the full N×N generalized gC and gL matrices that include the reference conductor.

Data structure

PULParameters gains two new fields alongside the existing ones:

mfem::DenseMatrix L, C;   // (N-1)×(N-1), SI units — unchanged
mfem::DenseMatrix gL, gC; // N×N generalized matrices, SI units — new

Computation

• gC — directly from getGeneralizedCMatrix(false) × ε₀
• gL — constructed algebraically from standard L via new helper buildGeneralizedL(), which produces an N×N symmetric matrix satisfying:
  • Row sums = 0 (closed-system charge/flux conservation)
  • gL(i,j) − gL(i,0) − gL(0,j) + gL(0,0) = L(i−1, j−1) for i,j ≥ 1 (standard L is exactly recoverable)

JSON output (example — 2-conductor coax, previously 1×1 only)

{
  "C":  [[8.02e-11]],
  "L":  [[4.67e-07]],
  "gC": [[ 8.02e-11, -8.02e-11], [-8.02e-11,  8.02e-11]],
  "gL": [[ 1.17e-07, -1.17e-07], [-1.17e-07,  1.17e-07]]
}

gC/gL are omitted when empty, keeping the format backward compatible with existing JSON consumers.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Export also generalized capacitance and inductance matrices</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Export also generalized capacitance and inductance matrices #44

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.