Update write_multi_stage_costs.jl with Updated and Corrected Discounting

This PR attempts to clean-up, update, correct, refine, and clearly document the multi-stage discounting of the costs.

src/multi_stage/write_multi_stage_costs.jl

@@ -36,7 +36,7 @@ function write_multi_stage_costs(outpath::String, settings_d::Dict, inputs_dict:
                     cum_stage_length += stage_lens[stage_counter]
                 end
             end
-            DF = 1 / (1 + wacc)^(cum_stage_length)  # Discount factor applied to ALL costs in each stage
+            DF_tech = 1 / (1 + wacc)^(cum_stage_length)  # Discount factor applied to ALL costs in each stage
         end
         df_costs[!, Symbol("TotalCosts_p$p")] = DF .* costs_d[p][!, Symbol("Total")]
     end