Added force calculation

src/pystencils/boundaries/boundaryconditions.py

+import abc
 from typing import Any, List, Tuple
 
 from pystencils.astnodes import SympyAssignment
@@ -5,11 +6,12 @@ from pystencils.boundaries.boundaryhandling import BoundaryOffsetInfo
 from pystencils.typing import create_type
 
 
-class Boundary:
+class Boundary(abc.ABC):
     """Base class all boundaries should derive from"""
 
     inner_or_boundary = True
     single_link = False
+    calculate_force_on_boundary = False
 
     def __init__(self, name=None):
         self._name = name