It should be possible to filter out relationships based on GCIs

This sounds quite obscure but is in fact extremely important to avoid erroneous propagation

consider

```yaml
[Term]
id: UBERON:0018140
name: mammary lobe
is_a: UBERON:0009912 ! anatomical lobe
relationship: part_of UBERON:0000310 {gci_relation="part_of", gci_filler="NCBITaxon:9606"} ! breast
relationship: part_of UBERON:0001911 ! mammary gland
```

this means that *in the context of humans* the mammary lobe is part of the breast. (in e.g. mice, there are mammary glands in other locations)

The OWL translation is here
https://owlcollab.github.io/oboformat/doc/obo-syntax.html#5.2.2

In OWL the meaning is very clear but there is a lot of possibility for mistranslation in obo when people ignore this field

it is important that applications are able to filter these relationships as naive traversal/propagation should *not* be used if the context is not satisfied

OBO Basic (https://owlcollab.github.io/oboformat/doc/obo-syntax.html#6.2) explicitly forbids GCIs due to possibility of mis-use

pronto is very useful for using with non-basic ontologies, but I don't see a way to filter on this?