graceful exit when there is no capacitance associated with a node

[onurbakir]

When C matrix has zero rows (such as when user accidentally defines resistive elements to disconnected ports) This matrix is not invertible. And in return julia returns a singularity error. Diagonal entries of this matrix (C should have a non zero diagonal if am I not mistaken) can be checked and user can be warned here.

AdmittanceModels.jl/src/pso_model.jl

Line 89 in 7b235d5

M = collect([C Z; Z I]) \ collect([-G -K; I Z])