proposal: accept unnamed streams

[cristianvasquez]

Currently, the service expects a carml:streamName "stdin" like this:

	rml:logicalSource [
		rml:source [
			a carml:Stream;
			carml:streamName "stdin"
		];
		rml:referenceFormulation ql:JSONPath;
		rml:iterator "$.characters[*]"
	];

If the name is not specified, the service returns 400.

However, CARML does not make this mandatory. Also, carml-jar expects stream-sources without streamName when processing through stdin

It would be nice if the service accepted mappings without streamName (and 'stdin' for backward compatibility)

Motivation: to make simpler CARML mappings, documentation, and XRM mappings. Also would be no mistmatch between carml-jar mappings and the ones that the service expects.