Add ST_Accessibility

Calculates, for each vertex, the (distance to the) closest destination among
several possible destinations.

#### Signatures

```mysql
SELECT * FROM ST_Accessibility('input_edges', 'o[ - eo]', 'ds')
SELECT * FROM ST_Accessibility('input_edges', 'o[ - eo]', 'dt')
SELECT * FROM ST_Accessibility('input_edges', 'o[ - eo]', 'w', 'ds')
SELECT * FROM ST_Accessibility('input_edges', 'o[ - eo]', 'w', 'dt')
```

where
* `input_edges` = Edges table produced by `ST_Graph` from table `input`
* `o` = Global orientation (directed, reversed or undirected)
* `eo` = Edge orientation (1 = directed, -1 = reversed, 0 = undirected). Required
  if global orientation is directed or reversed.
* `w` = Name of column containing edge weights as doubles
* `ds` = Comma-separated Destination string ('dest1, dest2, ...')
* `dt` = Destination table name (must contain column DESTINATION containing integer vertex ids)