allow to query by relationship type and return relationship type

I have been using this call to retrieve disease-phenotype associations from Monarch:

with urllib.request.urlopen("https://api.monarchinitiative.org/api/mart/disease/phenotype/NCBITaxon%3A9606") as url: disease_phenotype_data.append(json.loads(url.read().decode()))

It appears that there is no way to retrieve based on specified relationships and also the query result does not contain the relationship type.

Minimally it would be useful to return the relationship type. Querying by relationships is less useful potentially without a way of listing the available relationship types to begin with.

After discussion with @realmarcin, it would be also good to include disease labels in the response.

We can add an additional filter for relationship type. But for now you can treat these as a uniform relation 'has phenotype'