Models: FreeSurface Full-Stokes

[utkinis]

Finalise the model API

• Test 2D version
• Time-stepping
• Automate PT params creation

Export existing code for applying variational boundary conditions

• Compute volume fractions from Level Set (implemented, wrap with API)
• Weighted derivatives and null-space elimination for variational BCs (implemented, wrap with API)

New features:

• Sparse storage for volume fractions