nitmetrics: add --detect-covar

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add nitmetrics test for --detect-covar

Signed-off-by: Jean Privat <jean@pryen.org>
man: add --detect-variance-constraints in nitmetrics.md

Signed-off-by: Jean Privat <jean@pryen.org>