Returns a tibble of extracted estimates of \(\Sigma\) created by
the function niw.post() or siberMVN() in the packages
nicheROVER.
and SIBER.
The returned object will contain five columns in the
following order when data_format is set to "wide",
metric, id, sample_name, isotope, sample_number,
and the posterior sample for \(\Sigma\) (e.g., d13c and d15n).