suzerain: #2599 #2665 add density-squared stuff

Adds all of
    sym_rho2_grad_u
    rho2_grad_T
    rho2_u_u
    rho2_u_u_u
    rho2_T_u
    rho2_mu
    rho2_om
    rho2_om_om
which arise from looking at propagating uncertainty into Favre fluctuations.

Adds sampling to apps/perfect.

Adds relevant ignores to manual.gold.

git-svn-id: https://svn.ices.utexas.edu/repos/pecos/suzerain/trunk@44612 0bd85f0b-c151-43b9-a1e9-27231104551f