Crawler should be able to respect "obligatory" properties

Currently, the crawler relies on a misconfiguration of the server, because it needs to be able to "partially" insert "incomplete" entities (and complete them later...). This is a design issue.

There is already a planned solution:

• Collect all update and insertion commands and apply them at the end of the crawling process in one step. This solution, however, depends on the redesign of the whole crawler framework and might take some time.

Therefore, we should implement an intermediate workaround, using the force flag.

This bug will come into effect, as soon as this issue is resolved and the corresponding code is active: caosdb-server#146 (closed)

DoD

• Reproduced
• Checked whether tests already cover this; if they don't, extended them
• Added force flag to identifiable insertion