Implement missing LocalOperators from PDELab

The list:

• Convection Diffusion CG
• Convection Diffusion DG
• Convection Diffusion FV
• Navier Stokes DG
• Linear Acoustics DG
• Linear Elasticity
• Maxwell DG
• Nonlinear Convection Diffusion
• Taylor Hood Navier Stokes
• Mass operator