Better integration with iotables

[antaldaniel]

Similarly to 218 I would make a very cheap call and message to downloads of symmetric input-output tables. This affects about 12 products in the entire Eurostat catalgoue, however, I think it is important, because these special products should not be read as a data frame, but as a strictly formed matrix. They are meant to be used in input-output matrix equations. So the downloader with its long-form data frame is useless, and basically the entire point of the iotables R package on rOpenGov to make them useable.

So, I would add a simple check in the downloader function making sure that if these few codes are in the id parameter, the user gets a message the she should rather download this with the special downloader of iotables.

The iotables need not become a dependency of eurostat, therefore no extra weight is given to the package.

[antagomir]

Yes, really good.

[antaldaniel]

Included in PR 220