Add abstract Interface class as well as Point and Linear subclasses

interfaces.py

@@ -12,7 +12,13 @@ class Layer:
         return self.thickness / self.thermal_coefficient
 
 
-class Surface(ABC):
+class Interface(ABC):
+    @property
+    @abstractmethod
+    def HT(self) -> float: ...
+
+
+class Surface(Interface):
     def __init__(self, name: str, area: float):
         self.name = name
         self.area = area
@@ -26,6 +32,27 @@ class Surface(ABC):
         return self.U * self.area
 
 
+class LinearThermalBridge(Interface):
+    def __init__(self, name: str, length: float, psi: float):
+        self.name = name
+        self.length = length
+        self.psi = psi
+
+    @property
+    def HT(self) -> float:
+        return self.psi * self.length
+
+
+class PointThermalBridge(Interface):
+    def __init__(self, name: str, chi: float):
+        self.name = name
+        self.chi = chi
+
+    @property
+    def HT(self) -> float:
+        return self.chi
+
+
 class LayeredSurface(Surface):
     def __init__(
         self,