Add compute_switch_flows() to populate res_switch for zero-impedance bus-bus switches

Motivation

After runpp(), bus-bus switches with z_ohm=0 (the default) always have NaN in res_switch, since Pandapower fuses adjacent buses into a single PPC node. This makes it impossible to determine power flow through individual bus couplers, bus-section switches, or breaker-and-a-half arrangements.

Knowing cross-coupler flows is a common requirement in SCADA/EMS applications: real-time systems measure these flows via current transformers on the coupler path, and any simulation or state estimation that drives an SLD needs to reproduce them. Detailed substation models from CGMES/CIM imports represent the full busbar topology with explicit switching devices, and users expect res_switch to reflect the actual flow through each device.

Setting z_ohm to a small positive value is the existing workaround, but it causes convergence failures on large networks. We observed this consistently with 12,000+ bus-bus switches -- the Jacobian becomes near-singular from the many very-low-impedance branches.

Solution

New toolbox function compute_switch_flows(net), called after a converged load flow:

1. Identify fused-bus groups from the internal bus lookup.
2. Calculate net local injection per bus from res_* tables (loads, generators, shunts, wards, outgoing branches).
3. Build the switch tree within each fused group. Raises ValueError if a cycle is detected (flow split is physically indeterminate without impedance).
4. DFS from leaves to root to accumulate subtree demand and derive individual switch flows.
5. Write p_from_mw, q_from_mvar, p_to_mw, q_to_mvar, i_ka, and loading_percent into net.res_switch.

Switches with z_ohm > 0 are not modified.

Usage

import pandapower as pp
from pandapower.toolbox import compute_switch_flows

pp.runpp(net)
compute_switch_flows(net)
net.res_switch  # now populated for z_ohm=0 bus-bus switches