* previous DiagonalMatrix expression is now DiagonalMatrixWrapper
* DiagonalMatrix class is now for storage
* add the DiagonalMatrixBase class to factorize code of the
two previous classes
* remove Scaling class (it is now a global function)
* add UniformScaling helper class
(don't use it directly, use the Scaling function)
* add the Scaling global function to simplify the creation
of scaling objects
There is still a lot to do, in particular about DiagonalProduct for which
the goal is to get rid of the "if()" in the coeff() function. At least
it is not worse than before ! Also need to uptade the tutorial and add more doc.
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