Inspect filters to see if they can be translated a better execution model.

```sparql
PREFIX rdf:<http://www.w3.org/1999/02/22-rdf-syntax-ns#> 
SELECT 
  ?seq 
WHERE {
  ?x rdf:value ?seq . 
  FILTER(strlen(?seq) >5)
}
```
materializes the sequence as python string. Instead of using the `odgi.get_length(handle)` method.
If we could push such filter constraints into the `triples` method we would be able to be faster by generating less intermediate objects.