Support extension of existing needs

Support the possibility to add/overwrite options of existing needs.

For example,

.. comp-sys::
   :id: COMP_MY_COMPONENT

.. needs-extend:: 
   :filter: id==COMP_MY_COMPONENT
   :derives: REQ_SYS_1

[danwos]

Thanks for this really helpful discussion and all the ideas. They helped a lot to finally come up with a final spec. As I needed to make some tests to prove, if some details are possible in sphinx, there is already some really rough documentation (on feature branch only) and a first implementation of the directive:

As you can see, the idea from @Thomas-Swindells-t with the +-syntax got a revival, but on the option level.

So deleting a complete option (-) and adding additional input to an option (+) are supported. Modifying single values will not be supported in the initial version. Because I’m also not sure that there is a strong use case for this and that no simple workaround is possible.

Please let me know, what you think. Will finalize this in the next days for upcoming release 0.7.0

[danwos]

Right, checking this constraint must be done by the user and I think using needs_warnings is the right way.

We may think about a better modification monitoring. E.g. we could count which options got changed how often. So two different needextend on the same need would be okay, if they have modified different options. But I’m not sure, if this is a real or overthought use case…