Add maker method to MetricGaussianKL

ChangeLog.md

demos/getting_started_3.py

demos/getting_started_5_mf.py

demos/mgvi_visualized.py

src/minimization/metric_gaussian_kl.py

src/multi_field.py

src/operators/operator.py

src/sugar.py

test/test_kl.py

test/test_mpi/test_kl.py

test/test_sugar.py