Implement *_table_get_row

[gregorgorjanc]

Now that we have node_table_get_row in #132, it would make sense to implement *_table_get_row method for all tables. Seems very doable. It would also enable us testing of the *_table_add_row methods - do we get back what we have put in the tables.

[gregorgorjanc]

At the moment, the node_table_get_row works on TableCollection only, but in Python we can get a row of tables stored both in TableCollection and in TreeSequence and C method is for the node table (https://tskit.dev/tskit/docs/stable/c-api.html#c.tsk_node_table_get_row) not for TableCollection or TreeSequence. So, how should we address this in RcppTskit?

Any suggestions?