Node sets and side sets input/output

[hillyuan]

Dear repository owner

I am sorry I cannot find where REAME.md wrong and thanks for your modification.

Besides above pull request, I have also completed the implementation of Node sets, Side sets input from Abaqus inp file and output to exodus just now. I pushed it into my dev branch but I am not going to apply pull request because it is far from complete. I will stop this modification at this point, beacuse

1. I don't know what is your plan and what's your design.
2. There are too many works left to implement this function on all those mesh formats

It is basically needed and I think it is better to consider its implementation.

[nschloe]

If I understand you correctly, this issue is about how to handle node sets and side sets, and how to incorporate them in meshio. That is indeed something to consider.

Basically, this goes back to https://github.com/nschloe/meshio/issues/198 where the implementation of a container class is discussed. With this, one would read like

mesh = meshio.read(filename)

and mesh has mesh.x, mesh.cells etc. Additional data like sets could be stored in that object as well.

[nschloe]

Alright, I've just now released meshio 2.0.0 with a Mesh container class. You can simply add node/side sets to that class and add i/o for Abaqus/Exodus.