Rename problem file 'wriggers-l-shape' to 'identity-deformation'

Because that is what it is: It implements the identity deformation for three-dimensional continua.

The name 'wriggers-l-shape' refers to a simulation that appeared in

O. Sander, P. Neff, and M. Bîrsan: Numerical Treatment of a Geometrically Nonlinear Planar Cosserat Shell Model, (2016)

It belongs to the dune-gfe module, and only got into dune-elasticity by accident.