Disk Cache Class: Store and retrieve cache data in disk files

This class can store and retrieve cache data in disk files.

It can store a given data variable serialized in a cache file in a given directory.

The class can also check if the cache file exists and it did not expire, so it can retrieve its contents and return the unserialized data.

Existing cache files may also be explicitly deleted.

The path of the cache file directory and the expiry time are configurable parameters.