- set mu_c to 0
- introduce the shear correction factor 'kappa'

[[Imported from SVN: r8328]]

[[Imported from SVN: r8327]]

[[Imported from SVN: r8326]]

[[Imported from SVN: r7546]]

[[Imported from SVN: r7432]]

using the ideas of Patrizio Neff.

In particular, this code should handle real 3d continua, as well
as Neff-style Cosserat shells.

This initial commit only adds a lot of infrastructure.

[[Imported from SVN: r7339]]

[[Imported from SVN: r5726]]