Make sure formula in Sequence is callable (#52)

* 将序列的函数改为自定义函数

函数规格 
```python
def (x) # 返回第x个

* Update sequence.py

* Update utils.py

* Update sequence.py

* Update sequence.py

* fix code style and indent, remove useless line.

* remove extra spaces

Author: william-song-shy

cyaron/sequence.py

@@ -9,6 +9,8 @@ def __init__(self, formula, initial_values=()):
             Create a sequence object.
             int formula(int, function) -> the formula function ...
         """
+        if not callable(formula):
+            raise Exception("formula must be a function")
         self.formula = formula
         if list_like(initial_values):
             self.values = dict(enumerate(initial_values))