feature: timed measurements

There's already a pull request for timestamp instrument [#79](https://github.com/qdev-dk/Qcodes/pull/79) to add time to a measurement but it would also be useful to be able to use time in a Loop to trigger a measurement so that a parameter could be measured periodically. It seems to me this isn't covered at the moment given that measurement takes some time so using `qc.Wait(5)` doesn't actually guarantee a measurement every 5 seconds for example. I'm opening this as a discussion as to how this might be implemented and also if anyone else had thoughts/requirements for this.

Start by reviewing the Loop timing behavior and qc.Wait(5), then read pull request #79 for the related timestamp-instrument work. The issue is framed as a discussion and does not define a specific implementation or completion test, so requirements would need to be agreed before work can be considered done.

Written by the indexing model from the issue text.