Catch wrong user input

* python3 heat.py (without argument)
* python3 heat.py -d -n (with both arguments, while only one is allowed)

partitioned-heat-conduction/fenics/heat.py

@@ -54,11 +54,13 @@ def determine_gradient(V_g, u, flux):
 
 
 parser = argparse.ArgumentParser(
-    description='Solving heat equation for simple or complex interface case')
-parser.add_argument("-d", "--dirichlet", help="create a dirichlet problem",
-                    dest='dirichlet', action='store_true')
-parser.add_argument("-n", "--neumann", help="create a neumann problem",
-                    dest='neumann', action='store_true')
+    description="Solving heat equation for simple or complex interface case")
+command_group = parser.add_mutually_exclusive_group(required=True)
+command_group.add_argument("-d", "--dirichlet",
+        help="create a dirichlet problem", dest="dirichlet",
+        action="store_true")
+command_group.add_argument("-n", "--neumann", help="create a neumann problem",
+        dest="neumann", action="store_true")
 
 args = parser.parse_args()