Stop using strings in rooted_trees.py

[ketch]

We need a symbolic algebra system that can handle two multiplication operations. One possibility is to use Sympy's TensorProduct.

[ranocha]

A possibility to represent rooted trees that I like is using level sequences as described in the paper Constant Time Generation of Rooted Trees. I've used this form for the Julia package RootedTrees.jl and am quite satisfied with it.

[ketch]

That is a more computer-oriented representation of trees, and I would use it if I were implementing this from scratch now.

But this issue is actually intended to refer to something entirely different, which is the use of strings to represent the product on trees, in order to generate order conditions. See for instance lamda_str(). The reason for using strings there was just the need to represent a non-commutative product.

[ketch]

It looks like np.einsum is now the right solution for this.