[eigen] Specializing max_coeff_visitor for some number types |

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Hello,

`as far as I understand, Eigen uses max_coeff_visitor internally to
``determine the best pivot to use. And its notion of "best" is the largest
``in absolute value, which makes a lot of sense for float/double. For other
``number types, I may have a different notion of best. For example, for a
``rational, I may want to pick the "simplest" non-zero coefficient, to avoid
``an explosion in coefficient size. For interval arithmetic, I may want to
``compare only the lower bounds of the absolute values, or maybe involve the
``width somehow, but if we call operator< on [.9,1.1] and [.8,1.2], we have
``lost.
`

`As a quick hack, it looks like I can specialize scalar_abs_op to return
``something other than the absolute value, but that's ugly and I have to
``hope that abs is not used anywhere else in Eigen for other purposes.
`
Any advice?
--
Marc Glisse