Graph from a table of ontology relations

## Arguments

- relations
A data frame of ontology relations (the "relations" element of the list returned by

`obo_parser`

in case its argument `tables` is`TRUE`

).

## Details

By default the relations point from child to parents, the edges in the
graph will be of the same direction. Use `swap_relations`

on the data frame to reverse the direction.

## Examples

```
go <- get_db('go_basic')
go_graph <- relations_table_to_graph(go$relations)
```