Change the dataclass Interface into a regular class Surface.

Done for all subclasses of Interface too. This is a preparation for
later abstraction.

interfaces.py

@@ -12,21 +12,33 @@ class Layer:
         return self.thickness / self.thermal_coefficient
 
 
-@dataclass
-class Interface(ABC):
-    name: str
-    area: float
+class Surface(ABC):
+    def __init__(self, name: str, area: float):
+        self.name = name
+        self.area = area
 
     @property
     @abstractmethod
     def U(self) -> float: ...
 
+    @property
+    def HT(self) -> float:
+        return self.U * self.area
 
-@dataclass
-class LayeredInterface(Interface):
-    layers: list[Layer]
-    r_surface_1: float
-    r_surface_2: float
+
+class LayeredSurface(Surface):
+    def __init__(
+        self,
+        name: str,
+        area: float,
+        layers: list[Layer],
+        r_surface_1: float,
+        r_surface_2: float,
+    ):
+        super().__init__(name, area)
+        self.layers = layers
+        self.r_surface_1 = r_surface_1
+        self.r_surface_2 = r_surface_2
 
     @property
     def U(self) -> float:
@@ -35,9 +47,10 @@ class LayeredInterface(Interface):
         return 1 / R
 
 
-@dataclass
-class FixedUInterface(Interface):
-    u_value: float
+class FixedUSurface(Surface):
+    def __init__(self, name: str, area: float, u_value: float):
+        super().__init__(name, area)
+        self.u_value = u_value
 
     @property
     def U(self) -> float: