Reorganize functions

.gitlab-ci.yml

CMakeLists.txt

include/csg.hpp

include/solver.hpp

meson.build

src/csg/CMakeLists.txt

src/csg/csg.cpp

src/csg/levelset_2d.cpp

src/csg/levelset_3d.cpp

src/csg/matrix_functions.cpp

src/csg/meson.build

src/csg/parser.cpp

src/csg/tests/CMakeLists.txt

src/csg/tests/levelset/extrude.t.cpp

src/csg/tests/levelset/transform.t.cpp

src/csg/tests/meson.build

src/csg/tests/unit_tests_csg.main.cpp

src/csg_matrix_functions.hpp

src/csg_types.hpp

src/inputs/meson.build