Surface terms for ideal MHD

[Kostaszki]

In order to use nonperiodic boundary conditions we need to implement surface terms for ideal MHD.

[simonius]

I finished a raw draft of the surface terms, but i have some questions concerning the meaning of some variables. Anyway i hope to upload a beta next week.

[ranocha]

A first version has been implemented in #28. Other possibilities can/should be added as well.

[simonius]

I will try more complex speeds during the next weeks.

[ranocha]

The issue at the boundaries might be due to the nonconservative source terms introduced for stability. We should implement corresponding corrections and test them.
There are also some issues with time dependent boundary conditions and Runge-Kutta schemes, but I don't know whether they might cause the artifacts we see.
I hope to be able to reserve some time for this issue next week.