[[Imported from SVN: r1524]]

[[Imported from SVN: r1506]]

Add a method to assemble the Hessian matrix by a finite difference
approximation.

[[Imported from SVN: r1504]]

fixed one indexing bug in the hessian computation; added various helper methods for the derivatives of the strains and fd approximations for them

[[Imported from SVN: r1501]]

[[Imported from SVN: r1495]]

[[Imported from SVN: r1494]]

[[Imported from SVN: r1471]]

[[Imported from SVN: r1220]]

[[Imported from SVN: r1206]]

[[Imported from SVN: r1116]]

bugfix: the explicit expression for the Darboux vector given by Dichmann is already in local coordinates

[[Imported from SVN: r1105]]

[[Imported from SVN: r1093]]

[[Imported from SVN: r1092]]

[[Imported from SVN: r1091]]

[[Imported from SVN: r1089]]

[[Imported from SVN: r1088]]

[[Imported from SVN: r1087]]

[[Imported from SVN: r1086]]

[[Imported from SVN: r1073]]

[[Imported from SVN: r1060]]

[[Imported from SVN: r1048]]

[[Imported from SVN: r1040]]

[[Imported from SVN: r907]]

[[Imported from SVN: r852]]

[[Imported from SVN: r850]]

- Factorize derivatives of directors in separate method
- sign fixes

[[Imported from SVN: r845]]

[[Imported from SVN: r840]]

[[Imported from SVN: r838]]

[[Imported from SVN: r837]]

[[Imported from SVN: r836]]

[[Imported from SVN: r834]]

[[Imported from SVN: r817]]

[[Imported from SVN: r754]]

[[Imported from SVN: r742]]

[[Imported from SVN: r740]]

[[Imported from SVN: r597]]

[[Imported from SVN: r532]]

[[Imported from SVN: r482]]

[[Imported from SVN: r410]]