contingency: Fix bug for SCLOPF with lower branch constraint

The bug only affects extendable lines, and breaks code rather than
going unnoticed.

Thanks to Clara Büttner for spotting the error and fixing it!

pypsa/contingency.py

@@ -256,7 +256,7 @@ def add_contingency_constraints(network,snapshots):
 
             flow_lower.update({(branch[0],branch[1],b[0],b[1],sn) : [[(1,network.model.passive_branch_p[b[0],b[1],sn]),(sub.BODF[sub._branches.at[b,"_i"],branch_i],network.model.passive_branch_p[branch[0],branch[1],sn])],">=",-sub._fixed_branches.at[b,"s_nom"]] for b in sub._fixed_branches.index for sn in snapshots})
 
-            flow_upper.update({(branch[0],branch[1],b[0],b[1],sn) : [[(1,network.model.passive_branch_p[b[0],b[1],sn]),(sub.BODF[sub._branches.at[b,"_i"],branch_i],network.model.passive_branch_p[branch[0],branch[1],sn]),(1,network.model.passive_branch_s_nom[b[0],b[1]])],">=",0] for b in sub._extendable_branches.index for sn in snapshots})
+            flow_lower.update({(branch[0],branch[1],b[0],b[1],sn) : [[(1,network.model.passive_branch_p[b[0],b[1],sn]),(sub.BODF[sub._branches.at[b,"_i"],branch_i],network.model.passive_branch_p[branch[0],branch[1],sn]),(1,network.model.passive_branch_s_nom[b[0],b[1]])],">=",0] for b in sub._extendable_branches.index for sn in snapshots})
 
 
         l_constraint(network.model,"contingency_flow_upper",flow_upper,branch_outage_keys,snapshots)