Server -- Need to have a way of querying point-in-time data for a particular data source about cityxr-data-engine HOT 2 CLOSED

Added a latest variant of this as a starting point, which returns the most recent successful run.

Now, I'm not sure what happens if there are no successful runs, we should investigate.

Also, the return data format is the columns and then the data frames, to avoid ballooning that up too much.

from cityxr-data-engine.

Removed this acceptance criteria:

* How they want missing data filled in:
  - `clamp to most recent` meaning to show the most recent valid value for that data column
  - `lerp` meaning to do a time-weighted linear interpolation between the data column and its next valid reading

Reason being, we have datatypes that don't lerp very well (string, date).

from cityxr-data-engine.