suzerain: make the viscous filter the default one

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44407 0bd85f0b-c151-43b9-a1e9-27231104551f
suzerain: #3070 add TODO for discussion

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44408 0bd85f0b-c151-43b9-a1e9-27231104551f