File materialization functions

[mortenpi]

Conceptually, our datasets don't necessarily have an underlying file object on disk (e.g. the datasets fully stored in TOML files). As such, we only offer access to their contents via an ::IO object. However, sometimes you need to access it a File as a file system file (e.g. #60). We could export two file "materialization" functions:

• save(::File/FileTree, path::String): takes the File or FileTree object and writes it to path. The purpose of these functions is to allow the extraction of datasets from data repositories.
• materialize(::File): returns a file path; the purpose is to give (read) access to the dataset as a file system file. For datasets that already have a file somewhere on disk anyway, return that. For datasets that don't (e.g. TOML file, remote datasets), it would automatically create a temporary file (lifetime is the Julia session; would fix Better interoperation with Parquet files #60).