Make the linearized continuum NtD map handle arbitrary number of couplings.

This completes the implementation.  In principle, the Steklov-Poincare step
should now work for arbitrary configurations of rods and continua.

Known issues:
- rods must not stick to the same continuum with both ends
- all objects must have a nonempty Dirichlet boundary
  (will be fixed very soon)

[[Imported from SVN: r6856]]