Prevent heap allocation in diagonal product

Arthurrequested to merge
arthurfeeney/eigen:diag-prod-heap-alloc into master

Reference issue

fixes #2408 (closed) - product with a diagonal matrix had a heap allocation.

What does this implement/fix?

Just sets the NestByRefBit in the DiagonalMatrix traits to prevent the heap allocation.
Since this bit wasn't set, the Product class was not using a reference type! Instead, it was copying the diagonal matrix.

Edited by Arthur

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