Dataset.subset interface badly explained, works only if headers are set

It took me quite a while to figure out how to use `Dataset.subset`.

The documentation states the interface is `subset(rows=None, cols=None)`, so my first assumption was to just pass two ints for the number of requested rows and columns, and when that didn't work I passed a list of column indices, but also to no avail.

Only after debugging I finally found out that I first need to define column headers for the Dataset instance and then pass a subset of these headers to `cols`.

In my opinion, this could be made more clearly in the documentation.
Also, is there a reason why headers are required and we cannot alternatively just pass column indices?

I would go ahead and try to implement that myself if you don't mind.