Possible bug in tree_where when tree has a single leaf

[FerranAlet]

I'm not entirely sure, but I suspect that tree_where is assuming that a pytree with a single leaf is an np array, which can lead to issues of "TypeError: Field elements must be 2- or 3-tuples,". In my case, I had a list of a single numpy array. I couldn't directly test my hypothesis because I was using tree_where through OSQP, but when I hacked my code for the primal variable in OSQP to have two leaves, the code worked.

[mblondel]

CC @Algue-Rythme

[Algue-Rythme]

Hi Ferran

Could you give me more details on the failure mode of this function ?

I am not sure to understand which input format would cause those errors.

[Algue-Rythme]

The issue should be closed after the merge of 164.

[mblondel]

Fixed by #164