fix examples (#21)

pina/problem/abstract_problem.py

@@ -39,8 +39,6 @@ class AbstractProblem(metaclass=ABCMeta):
         else:
             raise RuntimeError('different domains')
 
-
-
     @input_variables.setter
     def input_variables(self, variables):
         raise RuntimeError

pina/problem/timedep_problem.py

@@ -10,5 +10,5 @@ class TimeDependentProblem(AbstractProblem):
         pass
 
     @property
-    def temporal_variables(self):
+    def temporal_variable(self):
         return self.temporal_domain.variables