Add a `dual2real()` function

[Andreas Dutzler]

How should this function / method be named?

• derive
• derivative
• variation
• dual2real
• downstairs

def dual2real(A, like=None):
    "Return a Tensor with old-dual data as new-real values."
    
    ndual = like.ndual - len(like.shape)
    
    return tr.Tensor(
        x=A.δx, 
        δx=A.Δδx, 
        Δx=A.Δδx, 
        ndual=ndual,
        ntrax=A.ntrax - ndual
    )