psi: refactor to reduce code duplication. * liboctave/numeric/lo-specfun.cc (psi): reduce code duplication by creating Lanczos_approximation_psi() to be shared between the functions for complex and real input. Also invert the order of the for loops (to sum smaller numbers first, which will increase accuracy. * libinterp/corefcn/psi.cc: adjust tolerance for the tests.

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