[codex] Move covariance recovery into GaussianBayesTree and expose ISAM2 queries #4625

Job	Run time
ubuntu-clang-quaternions Release	20m 47s
ubuntu-clang-system-libs Release	20m 57s
ubuntu-clang-tbb Release	27m 14s
ubuntu-build-examples Release	21m 10s
ubuntu-no-unstable Release	14m 55s
ubuntu-clang-cayleymap Release	24m 42s
ubuntu-clang-deprecated Release	25m 40s
	2h 35m 25s

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