[Feature]: FEM Vertex Constraints

[Milotrince]

What feature or enhancement are you proposing?

The ability to apply a positional constraint on specific vertices on a soft object.

Motivation

Related issues and discussion:
#1091
#199
#738
#271

Potential Benefit

Can attach a soft object to a rigid one by adding vertex constraints to the soft object at the spots it would be attached to the rigid object. Opens up more options for different hybrid robots, like a soft end effector.

What is the expected outcome of the implementation work?

• Handle constraints in FEMSolver (Dirichlet boundary condition)
  • Explicit FEM -> Can directly set/fix pos?
  • Implicit FEM -> Lagrangian
• A reasonably straightforward-to-use interface to add/update constraints
• Tests

Additional information

I will be working on this, guidance is appreciated!