Hi,

I ran into what might be a bug with the eigendecomposition function eigen() in ITensors.jl. Depending on the order of the Indices in my ITensor, eigen() gives me an error or not. Here is a minimum working code that gives me an error:

```
using ITensors
i1 = Index(1); i2=Index(2); i3=Index(3);
T = randomITensor(i1', i2, i3, i2', i3', i1)
F = eigen(T) # ERROR: In eigen, indices must come in pairs with equal spaces.
```

(I am using eigen()'s default behavior that the primed indices are the "left" indices and the rest are the "right" indices.) Note that the order of Indices that I provide to T is important. For example, the code below does not produce an error:

```
using ITensors
i1 = Index(1); i2=Index(2); i3=Index(3);
T = randomITensor(i2, i3, i2', i3', i1', i1)
F = eigen(T) # no error
```

Any suggestions on how to avoid this issue or what could be going wrong?