Provide node flags access

[molpopgen]

We should provide a means for mutable and immutable access to these via slices.

[molpopgen]

Given that applications like forward sims may want to change node times, we should add a mutable node table reference that provides slice access and SliceIndex access.

[molpopgen]

Given that applications like forward sims may want to change node times, we should add a mutable node table reference that provides slice access and SliceIndex access.

The trick here is making sure that lifetimes are okay in client code. For example, the following would have to realize that lifetimes are okay:

f = tables.nodes_mut().flags()

If the above works, we can then add convenience functions in TableCollection.

[molpopgen]

The trick here is making sure that lifetimes are okay in client code. For example, the following would have to realize that lifetimes are okay:

f = tables.nodes_mut().flags()

If the above works, we can then add convenience functions in TableCollection.

None of this is needed! Much easier than I assumed.