Fix/ldg penalty

Adding penalty term to LDGNS and switching on the LDG penalty term. The coefficient for LDG penalty term is simply set to 1, it should be reviewed later on. The coefficient for LDGNS is proportional to viscosity/thermal conductivity and inversely proportional to element size. The LDG has been lightly restructured to allow for a general flux. This allows for a diffusion coefficient different than 1 for UnsteadyDiffusion, UnsteadyAdvectionDiffusion and ViscousBurger, for the AV flux computation to be moved in the AV class, and eventually to merge LDG and LDGNS together.

• cleanup LDGNS "upwinding"
• cleanup LDG "upwinding"
• move AV flux computation
• convergence study LDG
• convergence study LDGNS
• check shock capturing
• update tests
• update documentation