[eigen] Specializing max_coeff_visitor for some number types

[Marc Glisse <marc.glisse@xxxxxxxx>]

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