add FlowMap.to_olca output format

This would follow [openLCA schema FlowMap](https://greendelta.github.io/olca-schema/classes/FlowMap.html) like [fedelemflowlist.write_jsonld](https://github.com/USEPA/Federal-LCA-Commons-Elementary-Flow-List/blob/48a6ab08be3703bd7262443973147c65389512c3/fedelemflowlist/__init__.py#L69)