numpy的copy

numpy的copy函數和引用不用，引用的話當原變量改變，引用也改變，copy得到的變量則不受此影響。

Examples
    --------
    Create an array x, with a reference y and a copy z:
    
    >>> x = np.array([1, 2, 3])
    >>> y = x
    >>> z = np.copy(x)
    
    Note that, when we modify x, y changes, but not z:
    
    >>> x[0] = 10
    >>> x[0] == y[0]
    True
    >>> x[0] == z[0]
    False