MD's xmas present: EB Godunov

[marchdf]

Seems to work ok for a shock hitting a cylinder

[marchdf]

Some answers:

• yes it will play nice with the re-re stuff. I will add a test
• thank you for adding those mass conservation tests
• I will add documentation that you suggest
• yes, good point

TODO based on comments:

• [x] add re-re test
• [x] add documentation
• [x] add more test like some of the EB-* ones
• [x] include pc_low_order_boundary stuff