Optimize only specific modes in cp_als()

[ghbrown]

An argument called optdims of type List[int] is added to cp_als() to enable optimizing only the factor matrices of specific modes. This was not previously possible even by scripting around cp_als() since it checks for convergence only after each full sweep. Control flow was hardly altered (dimorder may now just be missing some entries), and the default is of course optimizing all modes.

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📚 Documentation preview 📚: https://pyttb--302.org.readthedocs.build/en/302/