maintain site-wide metrics

[ssfrr]

We should be able to create a continuous query that would aggregate data across the site to get things like site-wide average temperature, humidity, etc. We also might want to calculate site-wide min/max and stdev. From an API standpoint I think we'd link to this data from the site resource.

Use Cases:

1. In the sonification we often re-scale data values relative to the site min/max or mean/std, this way we wouldn't need to keep track of that client-side.

Open questions:

1. How often do we recalculate this?
2. Do we also want time-aggregates for these data streams? In that case do we want all possible combinations of time-aggregate functions and sitewide aggregate functions (e.g., do we need to have the daily max of the sitewide min?)

I'd be curious what other use cases people would have for this, which would drive the features.