Fix FwdTrans and HelmSolve operators implementation

Fix FwdTrans and HelmSolve operators implementation:

• Update FwdTrans and HelmSolve operators for Dirichlet boundary condition
• Update FwdTrans and HelmSolve operators for Robin boundary condition
• Update ConjugateGradient solver for Robin boundary condition
• Create a tests/init_fwdtransfields.hpp subclass to reduce code duplication.
• Create a tests/init_helmsolvefields.hpp subclass to reduce code duplication.
• Consistently add test for seg, quad, tri, hex, prism, pyr, and tet.
• Use Nektar++ for reference solution.
• Remove deprecated Helmholtz2D.cpp file.

Note: MR !48 (merged) and !49 (merged) should be merge first.