Improve battery & building graphs in tutorial (displaying flex-model fields)

[nhoening]

Follow up on this comment, the graphs in the toy tutorial can now include flex-model information like the power-capacity (defined here).

I would extend this by also giving the battery a soc-sensor, plotting that as well as the soc-max setting.

The first thing is to add these values to the sensors_to_show structure for the building (building_asset.sensors_to_show ), "Power flows" graph:

{"asset": building_asset.id, "flex-model": "power-capacity"},

Second, the battery asset should get a "state-of-charge" sensor (PT0M resolution, kWh unit). That sensors's ID should be added in the battery's flex model under "state-of-charge" as {"sensor": ID}.
We can then plot both the soc and the soc-max by adding these lines to battery.sensors_to_show:

{
     "title": "State of charge",
     "plots": [
          {"asset": battery.id, "flex-model": "soc-max"},
          {"asset": battery.id, "flex-model": "state-of-charge"},
     ]
}

• I would want to test this (make a sreenshot to check).
• The screenshot should probably replace an existing screenshot in our docs. Note: we have both tut/toy-schedule/asset-view-without-solar.png and one wiith solar, and they are also used on the documentation's front page. I'm fine with them having an extra SoC plot.