Some updates for mesh generation

[schnellerhase]

While reading up on the mesh generation I had quite a hard time to understand what was going on in generation.h. This contains some tidying up of the code and relocation of input checks to more central positions.

What this PR does not change but should be done in the future is (if PR gets accepted I'll make this a new issue)

• [ ] unify interface of box based mesh generation, i.e. combine create_interval, create_rectangle and create_box
  • the checks for valid parameters can then be performed in one central location
  • the geometries that are created are already in 3D space, but we do not make use of this. For example an interval mesh is currently only to be generated along the $x$-axis and not along an arbitrary line segment in 3D space. Same holds for the rectangle interface. Changing this, also allows for common data types, i.e. we will no longer need to handle point types of different dimensions.
• [ ] extend testing -> currently a lot of the provided functionality is not tested thoroughly .