standalone attribute for `Section` and `Property` objects.

Following discussion #93

* Introduce `standalone` attribute for `Section` and `Property` objects. If `parent` is not explicitly set, default `standalone` could be `yes`.
* If defined as standalone, the object should be immutable and not attachable to another object. Use case could be a similar sections in multiple experiments could reoccur, so one could copy one of those in different experiment metadata.