New 95-line file for mixed/coupled DOF systems:
- MixedDOF{DOFs}: composition of multiple entity-based DOF types
- Helper functions: is_mixed(), get_dof_types(), num_fields(), get_field_dof_type()
- Support for Stokes (velocity+pressure), 3-field formulations, Taylor-Hood
- DOF ordering: concatenated in field order
- Display formatting for mixed DOF types
Enables mixed interpolation methods with different DOF types per field.