sixDoFRigidBodyDisplacement BC entries.

not available from db.

Removing rhoInf from uncoupledSixDoF... patch, not needed.  Adding
optional rhoName entry to sixDoF... so no rhoInf entry is needed for
simulations that have a rho field, and require than a rhoInf value is
read for those that don't.

- use ® instead of ® in html templates - seems to work better with
  doxygen at the moment

- avoids them showing on related pages

- slightly better memory efficiency and file sizes
- consistent with library reader

This makes the solver more robust when the "Courant number" of the flame
propagation fluctuates.