Implement live plotting support

[shumpohl]

Right now QuMADA has no "proper" support for live plotting during a measurement.

The currently intended(?) way is to monitor the database file and update the plot when the data of the current measurement in there changes. There are tools that can do this, either small self-written scripts using the shipped plotting utils or with plottr. However, there is no standardized way to inform the plotting tool on how to plot the data.

I see the following options:

1. Select a blessed way to include plotting information/instructions in the QCoDeS database (maybe there already is one - I did not find any)
   • Stay in QCoDeS universe
2. Keep a data structure with the results in memory and pass it to the plotter (xarray seems to be the best choice)
   • Prepares usage of alternative data format
   • More performant / quicker plotting

I prefer option 2 with xarray data structures. I propose the following plan of action:

• Define wrappers/subclasses around qcodes' Measurement, Runner and DataSaver with the goal to get the data with all structured information.
• Define a live plotting sink interface
• Use wrappers in the scripts package
• Add documentation for live plotting