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

[[Imported from SVN: r1504]]

[[Imported from SVN: r1503]]

[[Imported from SVN: r1502]]

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: r1473]]

[[Imported from SVN: r1472]]

[[Imported from SVN: r1471]]

[[Imported from SVN: r1470]]

[[Imported from SVN: r1469]]

[[Imported from SVN: r1468]]

[[Imported from SVN: r1220]]

[[Imported from SVN: r1206]]

[[Imported from SVN: r1124]]

[[Imported from SVN: r1123]]

[[Imported from SVN: r1118]]

[[Imported from SVN: r1117]]

[[Imported from SVN: r1116]]

[[Imported from SVN: r1115]]

[[Imported from SVN: r1109]]

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

[[Imported from SVN: r1105]]

[[Imported from SVN: r1104]]

[[Imported from SVN: r1103]]

[[Imported from SVN: r1093]]

[[Imported from SVN: r1092]]

[[Imported from SVN: r1091]]

[[Imported from SVN: r1090]]

[[Imported from SVN: r1089]]

[[Imported from SVN: r1088]]

[[Imported from SVN: r1087]]

[[Imported from SVN: r1086]]

[[Imported from SVN: r1075]]

[[Imported from SVN: r1074]]

[[Imported from SVN: r1073]]

[[Imported from SVN: r1061]]

[[Imported from SVN: r1060]]

[[Imported from SVN: r1059]]

[[Imported from SVN: r1048]]

[[Imported from SVN: r1040]]