DirPaths

Sorry about the huge PR again.

The idea of these changes is to use the `DirPath/FilePath` classes from mundane. In order to do this I have done the following changes:

 - The `Repository` now returns `ReferenceIO`s when asked for `factset`, `snasphot`, `dictionaries`,...
 - A `ReferenceIO` encapsulates a `Store[ResultTIO]` and a `DirPath` that is relative to the store root path
 - A `ReferenceIO` can either represent a directory or a file. It is up to the user of the `ReferenceIO` to know what to expect (for example for a Hadoop job, it could be either a file or a directory)
 - The `ReferenceStore` object provides methods to read/write/list files/directories from a `Reference` as required by the various low-level "Store" classes
 - The list methods on "Store" take `DirPath` arguments that are supposed to be relative to the store root and return `FilePaths/DirPaths` that are also relative to the store root
 - There is a `fullDirPath` method on a `ReferenceIO` to access the full path of the reference (including the store path)
 - There is a `.toHdfs` method for `ReferenceIO`  returning a `Path` object for the full reference path
This is just a case of the calling code being `(_:Repository).featureStores` where the return type has now changed.