add a restart method to tbl

[machow]

Note that currently you should be able to use tbl._init() but there is a bug. It tries to delete the previously discovered tables from the object, but the table names include the datasets, so it throws a key error

[machow]

The _init method was documented, which seems okay. tbl exposes datasets as attributes, so it makes sense that its setup / methods would start with underscore.