Add test for Dirichlet boundary condition, tidy-up DirBndCond and NeuBndCond operators implementation, and RobBndCond operator

• Tidy-up DirBndCond operator implementation
• Tidy-up NeuBndCond operator implementation
• Add new RobBndCond operator for Robin boundary conditions
• Create a tests/init_dirchletfields.hpp subclass to reduce code duplication.
• Consistently add test for quad, tri, and hex
• Use Nektar++ for reference solution.