Add cython defs for tdvp_common to speed up those loops.
Trade some time complexity for space complexity in the excitations code.
Correct choice of sparse eigs() in _calc_lr_ARPACK().
Add a function to calculate the spectral gap in E.
Small cleanups. Return number of 'iterations' for _calc_lr_ARPACK().
Add calculation of fidelity as well as gauge and phase alignment to tdvp_uniform.
Add calculation of overlap to tdvp_sandwich.