Missing functions in meshtools #82

There aren't many functionalities implemented in meshtools right now. A few easy functions would be potentially useful:

• Generating a box. This is trivial and hasn't been done before because the native Nek5000 tool genbox does it, but having it in pymech would allow using it as a building blocks for more complex meshes.
• A function to map geometries, for example to map a [0, 1]×[0, 1] box to an arbitrary shape with four corners. This is also trivial and would allow for generating more complex meshes.
• Coons patches? (trivial using the two functions above)
• A function to generate the outside of a circle.
• Functions to add, delete or change boundary conditions for velocity, temperature and passive scalars.

More complicated and speculative:

• A function to generate a cylinder-box junction.
• Integrating my airfoils script in some way. Perhaps by generating a boundary-layer mesh and returning some kind of analytic representation of the outer boudary. Then, combined with the functions above it might be possible to write a tutorial for generating a complete airfoil mesh with a boundary layer and a few free-stream blocks.
• Getting inspired by Vitor's extrude_refine function to perform some kind of mesh refinement.

Feel free to add anything to this list!