Allow ST_Accessibility to accept a single destination

By request of @mlecoeuvre.

Note that

```mysql
ST_Accessibility('cormen_edges_all', 'directed - edge_orientation', 'weight', '5')
```

is equivalent to reversing the graph and using `ST_ShortestPathLength` as
follows:

```mysql
ST_ShortestPathLength('cormen_edges_all', 'reversed - edge_orientation', 'weight', 5)
```

The only catch is that the following columns switched:

       ST_Accessibility | ST_ShortestPathLength
          ----------------------------
                SOURCE <=> DESTINATION
          CLOSEST_DEST <=> SOURCE

I show this in a unit test.