Add GPL-3.0 text, add SPDX-FileCopyrightText tags.

COPYING

Makefile

guix.scm

include/atomic_theta_helper.h

include/common.hpp

include/common.tpp

include/cyclotomic_field.hpp

include/discriminant_form.hpp

include/pari.hpp

include/sl2z.hpp

libweil.pc.in

scripts/weil-get-automorphisms.sh

src/atomic_theta_helper.c

src/common.cpp

src/cyclotomic_field.cpp

src/discriminant_form.cpp

src/pari.cpp

src/sl2z.cpp

src/weil.cpp