Add tutorial examples using DG

[rzhangbq]

Is your feature request related to a problem? Please describe. A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] It seems that your solver implements DG method, but there is no tutorial examples that uses it. Could you please implement a tutorial example for it?

Describe the solution you'd like A clear and concise description of what you want to happen. A tutorial example that uses DG method.

Describe alternatives you've considered A clear and concise description of any alternative solutions or features you've considered. None.

Additional context Add any other context or screenshots about the feature request here. None.

[vdweide]

I'll have a look at it.