Add NLsolve as method for power flow calculation

src/PowerFlow.jl

@@ -53,12 +53,19 @@ end
 Calculates the AC power flow solution for the given NDD and updates it in the NDD.
 CAUTION: Overwrites any previous AC solution contained in the NDD!
 =#
-function calc_ac_pf!(network_data::Dict{String,<:Any})
-    ### Calculate AC-PF solution using PowerModels.jl and Ipopt
-    pf_result = run_ac_pf(
-        network_data, 
-        optimizer_with_attributes(Ipopt.Optimizer, "print_level" => 0)
-    )
+function calc_ac_pf!(network_data::Dict{String,<:Any}, method=:JuMP)
+    if method == :JuMP # calculate AC-PF solution using a JuMP model
+        pf_result = run_ac_pf(
+            network_data, 
+            optimizer_with_attributes(Ipopt.Optimizer, "print_level" => 0)
+        )
+    elseif method == :NLsolve # calculate AC-PF solution using NLsolve.jl
+        pf_result = Dict{String,Any}()
+        pf_result["solution"] = compute_ac_pf(network_data)
+    else
+        throw(ArgumentError("Unknown method $method."))
+    end
+
     update_pf_data!(network_data, pf_result, model=:ac) # update PF in NDD
     return network_data
 end
@@ -121,8 +128,7 @@ function calc_branchloads!(network_data::Dict{String,<:Any}; model=:ac)
     end
 
     update_data!(network_data, branchloads) # add branchloads to NDD
-
-    return branchloads
+    return nothing
 end
 
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