feat: Adds multi-step form to generate equivalent lengths for a project.

Sources/ViewController/Extensions/EquivalentLengthForm+extensions.swift

@@ -0,0 +1,46 @@
+import DatabaseClient
+import ManualDCore
+
+extension SiteRoute.View.ProjectRoute.EquivalentLengthRoute.StepThree {
+
+  func validate() throws(ValidationError) {
+    guard groupGroups.count == groupLengths.count,
+      groupGroups.count == groupLetters.count,
+      groupGroups.count == groupQuantities.count
+    else {
+      throw ValidationError("Equivalent length form group counts are not equal.")
+    }
+  }
+
+  var groups: [EffectiveLength.Group] {
+    var groups = [EffectiveLength.Group]()
+    for (n, group) in groupGroups.enumerated() {
+      groups.append(
+        .init(
+          group: group,
+          letter: groupLetters[n],
+          value: Double(groupLengths[n]),
+          quantity: groupQuantities[n]
+        )
+        )
+    }
+    return groups
+
+  }
+}
+
+extension EffectiveLength.Create {
+
+  init(
+    form: SiteRoute.View.ProjectRoute.EquivalentLengthRoute.StepThree,
+    projectID: Project.ID
+  ) {
+    self.init(
+      projectID: projectID,
+      name: form.name,
+      type: form.type,
+      straightLengths: form.straightLengths,
+      groups: form.groups
+    )
+  }
+}