Clean up cache dir at end of expedition

[j-atkins]

Since #269, a cache dir is written in the expedition directory, storing information such as a record of problems encountered and keeping track of which have been resolved etc.

However, if running virtualship run with --difficulty-level easy there is no need to preserve the cache dir at the end of the simulation. It holds no useful information and in fact only interferes if re-running the same expedition.

Therefore, this PR adds a step at the end of a successful expedition simulation (when running in difficulty-level easy mode) to delete the cache dir.

• Tests