Create tiled mesh example function.

[BradWhitlock]

This PR includes a tiled() example mesh generation function that can take a 2D topology and stamp it out a bunch of times to make either 2D or (in most cases) 3D meshes. The tiled() function generates a single domain but can be used to make a multidomain mesh too with adjsets and boundaries. The mesh is generated using a square tiling of the tile topology but this can be put through a reorder() function that reorganizes the mesh points and elements. Reordering is currently done using a kdtree.

I added a conduit_generate_data utility that can invoke the tiled function to make a multi-domain mesh and save it to a file. It can also call braid(). There are times when I just want to make a mesh with Conduit without having to write my own program.

This update includes a adjset::compare_pointwise function that compares adjsets across domain boundaries to make sure they result in the same point on both sides of the domain boundary. I wrote it because I was messing things up when I built my adjset.

I added some sphinx doc for the tiled() example function and updated the CHANGELOG.md file too.

[cyrush]

Nice work!
Thanks for the new example, great docs, and refactoring the utils for broad use.
Found a case where we may want extra type logic for float32 and float64 (this case would be covered by double and float, but for consistency and completeness I think we should add)

[agcapps]

Thanks for the typo fixes. This work is very useful.

[cyrush]

thanks @BradWhitlock !