Minor changes related to GivensFGFT.

- Fix iteration error storing in GivensFGFT::errs,
- get_D() now returns D by ref instead of value (by copy),
- tests misc/test/src/Matlab/test_GivensDiag.m and misc/test/src/C++/GivensFGFT.cpp.in updated with their data input/output file misc/data/mat/test_GivensDiag_Lap_U_J.mat. Especially to test more things between C++ impl. and matlab original prototype (diagonalization_givens.m) :
- compare Dhat from matlab and C++,
- print iteration errors from matlab and C++,
- print more clearly the pivots comparison from matlab and C++.
- compare Uhat to U obtained by eig() matlab function (comparison in both cases: matlab prototype and C++ impl).