add LDDsimulation._eval_sources() method

LDDsimulation.py

@@ -135,6 +135,7 @@ class LDDsimulation(object):
         self._init_interfaces()
         self._init_subdomains()
         self._init_initial_values()
+        self._eval_sources()
 
     ## Private methods
     def _init_meshes_and_markers(self, subdomain_def_points: tp.List[tp.List[df.Point]] = None,#