Expose tables under topology for 2.x

[IAlibay]

Related to #4748

Expected behavior

In a non-API breaking release, I would expect to find the things I used to use in the same place, unless it was a bug and/or a DeprecationWarning had been raised for a while.

Actual behavior

MDAnalysis.topology.tables no longer exists in the current develop.

Proposal

We add it back with a warning.

[lilyminium]

+1, sounds good to me, following the same logic as in #4751

[orbeckst]

Can we also add a DeprecationWarning, given that it will go away and people seem to be using it directly?