feat: Adds psi to feet of head conversion.

Sources/ApiController/Extensions/FeetOfHead.swift

@@ -0,0 +1,38 @@
+import Dependencies
+import Logging
+import PsychrometricClient
+import Routes
+
+public extension FeetOfHead.Request {
+
+  private static let specificGravity = 1.02
+
+  func respond(logger: Logger) async throws -> FeetOfHead.Response {
+    @Dependency(\.psychrometricClient) var psychrometricClient
+
+    try validate()
+    let waterTemperature = DryBulb.fahrenheit(self.waterTemperature ?? 60)
+    let density = try await psychrometricClient.density.water(waterTemperature)
+    let feetOfHead = pressure / ((density.value / 144) * Self.specificGravity)
+    var warnings = [String]()
+
+    if self.waterTemperature == nil {
+      warnings.append(
+        "Calculations are based on 60°F water temperature."
+      )
+    }
+
+    return .init(feetOfHead: feetOfHead, density: density, warnings: warnings)
+  }
+
+  private func validate() throws {
+    guard pressure > 0 else {
+      throw ValidationError(message: "Pressure should be greater than 0.")
+    }
+    if let waterTemperature {
+      guard waterTemperature > 32 else {
+        throw ValidationError(message: "Water temperature should be above freezing.")
+      }
+    }
+  }
+}