pass dim to pygmsh.generate_mesh in 2d examples

[gdmcbain]

Not a big thing.

[kinnala]

What does it change?

[gdmcbain]

The (quite reasonable) default for pygmsh.helpers.generate_mesh is

https://github.com/nschloe/pygmsh/blob/78a32cd6711d158d0c9f27228f45b7e8f163b976/pygmsh/helpers.py#L80

and the gmsh executable is passed -{}.format(dim) which causes it to run through one-, two-, and three-dimensional meshing; e.g. (on master, before merging this)

python docs/examples/ex12.py

produces standard output

Info : Running 'gmsh -3 /tmp/tmpm0n09x1d.geo -format msh -bin -o /tmp/tmpq5r3twa2.msh' [Gmsh 4.2.3-git-7bc72f5de, 1 node, max. 1 thread]
Info : Started on Fri Mar 22 09:40:54 2019
Info : Reading '/tmp/tmpm0n09x1d.geo'...
Info : Done reading '/tmp/tmpm0n09x1d.geo'
Info : Meshing 1D...
Info : Meshing curve 1 (Circle)
Info : Meshing curve 2 (Circle)
Info : Meshing curve 3 (Circle)
Info : Done meshing 1D (0.00205061 s)
Info : Meshing 2D...
Info : Meshing surface 5 (Plane, Delaunay)
Info : Done meshing 2D (0.0160051 s)
Info : Meshing 3D...
Info : Done meshing 3D (8.4368e-05 s)
Info : 303 vertices 606 elements
Info : Writing '/tmp/tmpq5r3twa2.msh'...
Info : Done writing '/tmp/tmpq5r3twa2.msh'
Info : Stopped on Fri Mar 22 09:40:54 2019

and the user might puzzle over seeing

Info : Meshing 3D...
Info : Done meshing 3D (8.4368e-05 s)

when they know that their problem is two-dimensional.

Not a big thing. Is it worth typing dim=2 to avoid this? I don't know.