Nonlinear behavior for RelConnectsSurfaceEdge

[MartinSalak]

Similarly to #95 which introduced Nonlinear behavior for hinges on 1D. We would like to extend hinges on 2D the same way. This means adding the possibility to describe nonlinear behavior by assigning a NonlinearFunction to the hinge.

Have a look at already implemented RelConnectsStructuralMember which is already in the latest docs here:
https://www.saf.guide/en/latest/supports-and-hinges/relconnectsstructuralmember.html

Existing object: RelConnectsSurfaceEdge
Changed rows:

Property	Example	Definition
ux	Free, Rigid, Flexible, Nonlinear	Translation in X direction. Free - That is it imposes no constraint in the direction. Rigid - The connection in fully rigid in the specified direction. Flexible - The connection is flexible (elastic) in the specified direction. Parameter Flexible can be linear only, non-linearity is not supported. Nonlinear – The connection’s behavior is set by a function
uy
uz
fix
fiy
fiz
Stiffness X [MN/m]	100	Now required for ux = Flexible or Nonlinear. For Nonlinear it will be used for linear analysis and it can be used as a starting point for nonlinear analysis.
Stiffness Y [MN/m]
Stiffness Z [MN/m]
Stiffness Fix [MNm/rad]
Stiffness Fiy [MNm/rad]
Stiffness Fiz [MNm/rad]

New rows:

Property	Example	Definition
Function ux	Press only	Reference to object NonlinearFunction, applicable if ux = Nonlinear. Nonlinear function must be of type Translation
Function uy
Function uz
Function fix	Nonlin fun 3	Reference to object NonlinearFunction, applicable if fix = Nonlinear. Nonlinear function must be of type Rotation
Function fiy
Function fiz