Revisiting the evolution eigensolver.
This commit contains few bugfixes: - The Taylor expansion for imaginary time was not correct. - The code was wrongly mixing the density between timesteps of the evolution - The code was doing a subspace diagonalization which is not correct - The orthonormalization routine was not stable and is replaced by the standard orthogonalization. - The code was freezing states once converged, which is not correct unless independent particles Besides this, the code now check that the exponential is stable. The default time-step is set to a valid close to the one shown in the litterature.
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