adding softlink

This adds an implementation of resolving SoftLinks. The variety of SoftLink that appears in SymbolTable.get_links has been tested and works, but I couldn't find any files that contain link messages (0x0002) in the tests or in my typical working files, so that has not been tested (the _get_links_from_link_msgs method in DataObjects)

Implementation notes: soft links are distinguished from hard links by the fact that they are strings, in the 'get' method in high_level.Group class. If a link is a string (and therefore a soft link) it is sent back to "get" as a string, and if the lookup fails None is returned, as in the spec.