Add optional send_count parameter to collectives

[Hespian]

Right now all operations seem to just send all data they get in their send buffer. Sometimes you may want to send only a part of that. Right now the only possibility to do so would be with a span.

[kurpicz]

For me this seems to be more of a feature than a bug. We have simple syntax to send any interval of the data (span) or the whole buffer (nothing needs to be added). I am not sure a send_count parameter is simpler. However, I have no hard feelings w.r.t. adding a send_count parameter.

[Hespian]

Maybe you are right and this would just bloat KaMPIng without adding much. Maybe we can just postpone this until we actually implement stuff in KaMPIng and see that it would actually be useful

[niklas-uhl]

It would also prefer using span for this. This is conceptually in line with #301. At the moment its a bit awkward to create a span, maybe we just add helpers for that and we're fine. Something like:

std::vector<int> v = { ... };
comm.send(send_buf(make_subview(v, 0, 32)), rank(1));  //make subview returns a span

[lukashuebner]

Imho, that's overly verbose, which is something we explicitly decided that we're trying to avoid. I'm strongly pro send_count.

[niklas-uhl]

This should be closed by #118