Data source cache method

[johngrimes]

It would be great if there was a cache() method on a DataSource object within the library API.

This would go through each of the datasets within the data source and invoke Dataset.cache() on it.

This would be desirable in some situations where multiple queries need to be run over the same data source.

[johngrimes]

This would make one of of the workarounds for #1770 easier.