[models] added "str" method to VariableType

pyheatpump/models/variable_type.py

             self.slabel = self.label[0]
 
 
+    def __str__(self):
+        return self.slabel or self.label[0]
+
+
     def select():
         try:
             elt = next(super().select('slabel', 'var_type'))
         except StopIteration:
             print('No element exists')
 
+
     def save(self):
         q = ['UPDATE var_type SET']
         updates = []