Feature/optimization symbolic expr degree

[vincentchabannes]

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• Optimization of the computation total degree of a symbolic expr (polynomial). Previously the algo complexity was clearly very bad ( double loop + recursion non-terminal). Now it's just an iterative version which is faster. For example, this computation was done with EIM->beta() online computation and the perf is huge.
• Reduce some log verbosity

[prudhomm]

warning could you keep int ?
I am making in another branch the polynomial degree dynamic like in Eigen (compile time or dynamic)

[prudhomm]

here is a discussion also on uint vs int http://eigen.tuxfamily.org/index.php?title=FAQ#Why_Eigen.27s_API_is_using_signed_integers_for_sizes.2C_indices.2C_etc..3F

[vincentchabannes]

I am not sure to understand why it's a problem because, here, it's a local variable used in the algorithm (and I need to check if value is invalid_v< uint16_type> or not. If really an issue, we can change to int and check non negative value.