This is actually what is claimed in the paper.  Previously, the mirror
axis was y=0.005.  Having the grid symmetric across the x-axis itself
simplifies the expressions for the boundary conditions a little bit.

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

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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.

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