Table collection returned by ts.tables not read-only.

[jeromekelleher]

One thing that has been bugging me is the description of mutability. If you have a tree sequence, calling the functions that change it do not raise an error (like the various add_row methods). Rather, they just silently do nothing.

Originally posted by @molpopgen in https://github.com/tskit-dev/tskit/pull/756#issuecomment-670972776

It would be good to do this, and the plan is for the table collection to be read-only so that we can avoid taking copies when creating a new set of tables. However, I think there would be a fair bit of work required to do this at the Python level, and we'll still want to actually mark things as read-only at the C level (so we can be sure things are safe). So, I think we should just tackle this properly when we do it.

[jeromekelleher]

Note: when we do this we should update the documentation for the arrays added in #2424 to take out the comment about ts.tables incurring a full table copy. There's also some other warnings scattered around the docs to find and remove.