_repr_html_ should be placed in Fetch

[eywalker]

Currently _repr_html_ is implemented in the RelationalOperand. This causes the fetch execution to occur when you typein the name of the RelationalOperand (and its derivatives) in IPython/Jupyter notebook. This is in disagreement with the rest of the repr behavior, where we only fetch and preview table content for repr in the Fetch object. Shall we move _repr_html_ into Fetch and Fetch1 to be more consistent with the rest?

[fabiansinz]

I guess we can close this issues because now both __repr__ and _repr_html_ cause a fetch operation. Even though this is not in agreement with the idea that only the fetch object should fetch data, it reduces the effort of typing .fetch everytime we would like to see some content of the table.

[dimitri-yatsenko]

Now RelationalOperand has the preview method. RelationalOperand.__repr__ calls preview() when not in debug mode.

Closing.