Handle Section 'strict' merge

[mpsonntag]

On a Property strict=True merge, all Property attributes except value, name and id have to be identical.

On a Section merge, the child Sections and Properties of the handed in Section are appended to the invoking Section, but no attribute checks are performed.

The question is, whether on a strict=True merge of a Section, any attributes e.g. definition should be identical on Section level as well to allow a merge. See #271 (comment).

Any comments or opinions?

[jgrewe]

imho, they should be the same, similar to the check done in Property. The same for repository and reference, methinks