Write meshtags on input mesh

[jorgensd]

(Looking at you @finsberg and @MariusCausemann) we can now write meshtags on input mesh.

[finsberg]

This is great @jorgensd! This is definitely the source of many bugs that I have encountered, and I am wonder whether it would make sense to make the default behaviour to use the input mesh (also for function)? I know that it will not be as performant, but IMO it is better to avoid bugs, than being a bit more performant 🙃

[jorgensd]

This is great @jorgensd! This is definitely the source of many bugs that I have encountered, and I am wonder whether it would make sense to make the default behaviour to use the input mesh (also for function)? I know that it will not be as performant, but IMO it is better to avoid bugs, than being a bit more performant 🙃

I refuse to change the default behaviour of the functions;

1. Changing defaults should have a very good reason
2. Degrading performance when the documentation clearly states what kind of checkpoints exists makes it kind of pointless to have documentation.
3. Now that we have write_point_data and write_cell_data, I think most use-cases for people will be reading in data from visualizable files rather than checkpointing files.

[finsberg]

Just one minor comment, otherwise feel free to merge.

[finsberg]

Perhaps we can just remove this, it is_complex always is False?

[jorgensd]

Sure, remainder from copying old test