Feature: ND variable covariance Gaussian likelihood

demos/a_NDVariableCovarianceGaussian.py

src/re/__init__.py

src/re/likelihood_impl.py

src/re/multi_grid/kernel.py

src/re/refine/charted_field.py

src/re/refine/healpix_refine.py

src/re/tree_math/__init__.py

src/re/tree_math/util.py

test/test_re/test_likelihood_impl.py