suzerain: add mean of species concentrations to stats

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44760 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #2998 substract mean term to get two-point correlations

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44761 0bd85f0b-c151-43b9-a1e9-27231104551f