suzerain: mark bug with FIXME

Man am I dumb.  E[rho u] != E[rho] * E[u]
Flagging these for now while I look around for other mistakes.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44971 0bd85f0b-c151-43b9-a1e9-27231104551f