Start to add new validation test for AC grids
Added first unit test of the AC one phase line (base for others). The commercial cable mode works and satisfy the analytical results when in short circuit and when cables are in parallel or series.
Other base tests for AC single phase lines
Piecewise linearization of the DC load. Significantly reduced the error. Kept linearity.
Added homotopy to linearized loads, simplified formulation is current i[:] = 0 (A)