Fix dataloader table names

[LLehner]

The ImageTilesDataset class cannot be used if the table you want to work with is not called "table", since there are lines where this name is hard-coded, even though the class allows for a table_name to be specified.

An example where this fails is if you follow this tutorial but specify a different table name when creating the dataset.

This PR replaces instances of "table" with table_name.