Convert every allOf tag to oneOf in OpenAPI schema

[RichardBruskiewich]

TRAPI 1.3 validation was problematic with jsonschema (inside the reasoner-validator) due to the use of allOf OpenAPI schema property alongside nullable: true.

Review of the definitions for allOf versus oneOf suggest this is a reasonable revision, given that logically, one cannot satisfy both a concrete non-null JSON data model and a null value, at the same time, but rather, choosing one or the other (i.e non-empty schema xor null, i.e. oneOf) is a more reasonable semantic expectation.

[edeutsch]

Addressed in #403