[eigen] nesting

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While looking into the "performance degradation" issue from the forum
I found out that it is due to temporaries - as Benoit already guessed.

I am a little bit afraid, that what I once proposed, namely copying
expressions by value, is now backfiring. The reason is that initially
I assumed expressions to be tiny little objects with close to no copy
costs. The issue is related to those expressions holding temporaries.
Copying them (e.g. a product expression) means copying all the data
including the temporary and that will happen as many times as we nest

The only solution I can think about at the moment is the
specialization of ei_nested for those types and to go back to nesting
by reference for these heavy weight guys.

- Hauke

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