Add specific capabilities and requirements to fibers and targets

Given the future non-availability of the `n2`-arm on one of the spectrographs, it was requested to preferrably assign only those targets to the affected fibers which don't require this part of the spectrum.

Following a suggestion by mxhf, this could be done by simply removing all pairings between the affected fibers with targets requiring the full spectrum. This requires an interface change - we have to add a property to targets that they do or don't require the `n2` arm and to Cobras whether their assigned `n2`-arm is working.

However, if we assume that similar issues will occur in the future, we might future-proof the fix by introducing a generic set of properties required by targets / provided by Cobras, so that we can avoid additional interface breaks whenever such an issue pops up.

I'm happy to work on the general solution, but I'd be happy to establish a consensus first, and it will take a bit of time.

Cf Masayuki TanakaWanqiu Heyuki.moritani