To replace Dokchitser's scripts.

some information here:

• https://icerm.brown.edu/materials/Slides/sp-f15-w3/L_functions_in_Pari-GP_]_Pascal_Molin,_Universit%C3%A9_Paris_Diderot.pdf

• https://pari.math.u-bordeaux.fr/dochtml/html/_L_minusfunctions.html

• http://pari.math.u-bordeaux1.fr/Events/PARI2016/talks/LCheat.pdf

• http://pari.math.u-bordeaux1.fr/Events/PARI2016/talks/L.pdf

CC: @JohnCremona @davidlowryduda

Component: number theory

Keywords: dokchitser, lseries, zeta

Author: Frédéric Chapoton

Branch/Commit: 091f71b

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/26098