feat: add kwarg to instrument constructor on server

[damazter]

Changes proposed in this pull request:
Add a kwarg to the constructor of instrument that contains a reference to the instrumentserver that created the instruments.
This will enable that all instruments on the same server can find each other via the instrument server and can talk to eachother

@giulioungaretti

add a reference to the instrument server that created the instrument

[damazter]

A usecase for this new kwarg can be found in #66

[qcodes-bot]

Current coverage is 82.82% (diff: 100%)

Merging #303 into master will not change coverage

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[AdriaanRol]

@damazter, Do I understand correctly that this means that an instrument (A) can find any other instrument (B) that lives on the same server (by its name) without the need for ins B to have existed at creation time (of ins A)?

If so I think this is a great addition as it removes the need for using shared kwargs as long as one ensures that the required instruments exist on the same server.

[damazter]

@AdriaanRol
To your first point: with this change you're exactly right except for the fact that the lookup cannot be done by name (as far as I know) but only by ID_number, not by name. This would be a really nice addition though.

Your second comment is correct as well, but this doesn't mean that the entire shared_kwargs framework becomes irrelevant, because you would still need that to communicate between different servers. (see http://qdev-dk.github.io/Qcodes/user/tutorial.html# at meta instrument)

[damazter]

@AdriaanRol

(by its name)

Looking at this again, I think it is possible to do it by name as well by doing something like

reference = none
for inst in server.instruments:
    if inst.name == the_name_you_want:
        reference = inst

[damazter]

superseded by #323