Add FixedRLayer and move the existing Layer into a hierarchy

interfaces.py

@@ -1,17 +1,31 @@
 from abc import ABC, abstractmethod
-from dataclasses import dataclass
 
 
-@dataclass
-class Layer:
-    thermal_coefficient: float
-    thickness: float
+class Layer(ABC):
+    @property
+    @abstractmethod
+    def R(self) -> float: ...
+
+
+class MaterialLayer(Layer):
+    def __init__(self, thermal_coefficient: float, thickness: float):
+        self.thermal_coefficient = thermal_coefficient
+        self.thickness = thickness
 
     @property
-    def R(self):
+    def R(self) -> float:
         return self.thickness / self.thermal_coefficient
 
 
+class FixedRLayer(Layer):
+    def __init__(self, r_value: float):
+        self.r_value = r_value
+
+    @property
+    def R(self) -> float:
+        return self.r_value
+
+
 class Interface(ABC):
     @property
     @abstractmethod