rename tables to _table for basic relationships

rename tables to _table for basic relationships

the names "parent" / "child" are confusing for new users
in that they are used for table names as well as
"back_populates='parent'", use a disambiguated name.

In this change, there's now overlap between the
variable named "association_table" and the table name
"association_table".  not sure of a better naming system.

Change-Id: Ic036c8072caf6e9e5fbd1178986353c00b91f43d
References: https://github.com/sqlalchemy/sqlalchemy/discussions/8675#discussioncomment-3915204