refactor templates data handling and http apart.
Introduce a download path into the pipeline.

This will generate a csv for download in the background. users will poll
a status URL to see if it's ready.
Implement filled measurements for downloads.
wire up route with pipeline for csv download.
add some tests.

Stepping through the sequence of sequence of measurements is complicated
code. these tests do some validation on simple input data and also does
a generated test to make sure that no input breaks the code at
least. The generated test doesn't validate the output, thats for another day.
http wiring, date handling.