Add minimal example for the computation of bending isometries.
The Merge Request adds the following features related to the computation of bending isometries:
- bending-isometries.cc (source file for bending isometry experiments).
- A header file 'discretekirchhoffbendingenergy.hh' defining the Kirchhoff bending energy.
- A header file 'bendingisometryhelper.hh' containing various related helper functions.
- A header file 'localdiscretekirchhoffbendingisometry'. This is used for the evaluation of deformations and serves as 'LocalInterpolationRule' for the LocalIntegralEnergy-class.
- An additional minimal experiment 'L-clamped-Plate.py' computing the deformation of a square domain that is clamped on an L-shaped part of the boundary.