Major caching update for SpaDES

[Eliot McIntire]

Dear All,

I just pushed a major set of updates for Caching in SpaDES, including a major update to the help document describing caching ... ?cache

1. Deal correctly with S4 methods and Rasters. The previous version of cache stumbled under many fairly common situations (Rasters, S4 methods). To defend my earlier version, I had assumed that the archivist version would have handled S4 methods, but it doesn't.
2. Cache function. I added a function Cache (upper case) that doesn't require cacheRepo to be specified if used inside a module, and it doesn't conflict with archivist::cache, so you can just write Cache(    ) instead of SpaDES::cache(  )
3. Module Caching. I added a major new functionality... a whole module can be cached now if the parameter .useCache is TRUE. 
4. Cache help file. Major update to describe the 3 levels that currently can be used (model level [i.e,. spades call], module-level, function-level)

If you find that it is not working as anticipated, please try to describe it (and make a reproducible example if possible), but it should be pretty good.

[Alex Chubaty]

Just to clarify, this change has been made in the development branch only. It is not yet in the master (CRAN) version of the package.