make it easier to add entries to the replica catalog by inferring site, lfn, pfn from file Path or URL

Suggested by Rajiv, the general idea is this:
# create a file objects
a = File(Path("/dir/f.txt")
b = File("http://hostname/f2.txt")

standard file paths assume site to be "local", for any other URL type, site does not matter

rc = ReplicaCatalog()
rc.add(a,b)
# this would be the same as doing
rc.add_replica("local", "f.txt", "/dir/f.txt")
rc.add_replica("anysite", "f2.txt", "http://hostname/f2.txt")

# but then convey the same amount of information with much less text