feat: Adds equivalent rectangular duct conversion.

Sources/ManualDClient/Helpers.swift

@@ -4,3 +4,9 @@ import ManualDCore
 extension ComponentPressureLosses {
   var totalLosses: Double { values.reduce(0) { $0 + $1 } }
 }
+
+extension Array where Element == EffectiveLengthGroup {
+  var totalEffectiveLength: Int {
+    reduce(0) { $0 + $1.effectiveLength }
+  }
+}