Rigid body connector

[lijas]

Rigid body connectors

It would be nice to support Rigid body connectors (RBE2 and RBE3) in Ferrite. They are quite common to use in industry, and there has been questions about them in the slack channel before. They are usually modeled with rigid bodies which are connected to some part of the mesh (for example the nodes/facets around a hole).

This PR adds:

1. Point<:AbstractCell and an interpolation which can be used to add rigid bodies dofs to the system.
2. A RigidBodyConnector<:Constraint (RBE2) which sets up the affine constraints between the rigid body and connecting mesh (This constraint is pretty specific for solid mechanics problems, so it should probably not be included in core Ferrite).
3. A simple example illustrating the constraint (a plate with a hole, and a rigid body at the center that is connected to the nodes around the hole. The rigid body is rotated pi/10).

[codecov]

Codecov Report

Attention: Patch coverage is 1.81818% with 54 lines in your changes missing coverage. Please review.

Project coverage is 90.78%. Comparing base (5153307) to head (be445f8).
Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Dofs/ConstraintHandler.jl	2.32%	42 Missing ⚠️
src/interpolations.jl	0.00%	7 Missing ⚠️
src/Grid/grid.jl	0.00%	4 Missing ⚠️
src/Export/VTK.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1119      +/-   ##
==========================================
- Coverage   93.57%   90.78%   -2.80%     
==========================================
  Files          39       39              
  Lines        6071     6260     +189     
==========================================
+ Hits         5681     5683       +2     
- Misses        390      577     +187     

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[KristofferC]

This constraint is pretty specific for solid mechanics problems, so it should probably not be included in core Ferrite).

Is there a way to formulate this constraint in a somewhat "general" way? I think it is fine to have in Ferrite even if the most common use case is for a certain class of problems.

[lijas]

I tried to think of way to generalize it, but i dont think there is an analog constraint for e.g. heat problems.

[termi-official]

I think rather than having this directly in Ferrite we should think about providing the building blocks (and howtos) to build such special constraints in an efficient way. In the end this just another affine constraint, since I think building most affine constraints is right now difficult for most users.

[lijas]

Yes, it could work as a how-to. It would also be fun to replicate this phase field problem, where they use these rigid body connectors to apply the boundary conditions.