reproducible: Enhance Reproducibility of R Code

A collection of high-level, machine- and OS-independent tools for making reproducible and reusable content in R. The two workhorse functions are Cache() and prepInputs(). Cache() allows for nested caching, is robust to environments and objects with environments (like functions), and has deals with some classes of file-backed R objects e.g., from terra and raster packages. Both functions have been developed to be foundational components of data retrieval and processing in continuous workflow situations. In both functions, efforts are made to make the first and subsequent calls of functions have the same result, but faster at subsequent times by way of checksums and digesting. Several features are still under development, including cloud storage of cached objects, allowing for sharing between users. Several advanced options are available, see ?reproducibleOptions().