Added a filesize check to local dataset cache

I ran into the problem where I was experimenting with my own dataset and local caching. When I updated the dataset, the `LocalDatasetCache` just compared filenames and used the outdated locally stored version. This can be pretty tricky to debug, because the model trains just fine, but it's not training on the data that you've given.

I'm not sure whether it's better to compare file sizes (fast, but no guarantee that the files are actually the same) or to use `filecmp`, which will actually do a byte-wise comparison of the two files (takes approx. 7 seconds on sencha for a 3.6GB file). I implemented the former here.