Add caching support for external data files

[danharrin]

Currently, Sushi models are only cached when the $rows property is used. This is because the alternative is usually to return data from an API using getRows(), and caching would mean this data could be out of date.

This PR introduces support to override that behaviour.

First of all, instead of only checking the model file for its last updated timestamp, return a different file path from getDataPath(). I frequently like to store Sushi data in config / CSV files instead and return from getRows(), and this feature allows the cache to know when it is out of date.

To allow these external sources to be cached, I've also introduced the isCached() method. This allows you to enable the cache at will when using getRows(), while preserving original behaviour.

Thanks!