getRelativeURL: Compute name of URL relative to a base URL
Description
This function is a convenience function for
computing the fullly qualified URI of a document
relative to a base URL.
It handles the case where the document is
already fully qualified and so ignores the base URL
or, alternatively, is a relative document name and so
prepends the base URL.
It does not (yet) try to be clever by collapsing
relative directories such as "..".
Usage
getRelativeURL(u, baseURL, sep = "/")
Arguments
u
the location of the target document whose fully qualified URI is to be determined.
baseURL
the base URL relative to which the value of u should be interpreted.
sep
the separator to use to separate elements of the path. For external URLs (e.g.
accessed via HTTP, HTTPS, FTP), / should be used. For local files on Windows machines
one might use .Platform$file.sep, but this is incorrect unless one