Use proxy types that are passed to P4estMesh to allow to dispatch on the first argument?

[efaulhaber]

This is more of a general remark. The P4estMesh has many different options of generating meshes: structured, INP, now cubed sphere. Possibly soon also UnstructuredQuadMesh. Instead of differentiating by the arguments to the constructor, what about using proxy types that are passed to P4estMesh that allows to dispatch on the first argument?

For example (names can/must be optimized),

• Abaqus
• Structured
• CubedSphere
  Such that you could just call

P4estMesh(CubedSphere(3, 2, 0.5, 0.2), ...)

I am not sure if this needs to be done in this PR, I just wanted to put this out as an idea.

Originally posted by @sloede in #689 (comment)