Get resampled data

[Flix6x]

This PR introduces the "resolution" field to /sensors/data (GET) to obtain data in a given resolution.

[coveralls]

Pull Request Test Coverage Report for Build 2656017059

• 2 of 3 (66.67%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.003%) to 68.367%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
flexmeasures/api/common/schemas/sensor_data.py	1	2	50.0%

Totals
Change from base Build 2637707449:	-0.003%
Covered Lines:	7097
Relevant Lines:	9846

---

💛 - Coveralls

[nhoening]

Looks clean.

I have a general question about testing.

[nhoening]

So here we're testing that timely beliefs changes the representation of the field.
Is there no need to use some data to test resolution adaptations? Do we already test that someplace else (testing the search function?

I noticed that in the API level we are not testing GETting sensor data, just posting...

[Flix6x]

Not timely-beliefs. This checks whether the given marshmallow schema in FlexMeasures parses these particular ISO durations as python timedeltas. I'd love to extend this with some nominal durations, but unfortunately timely-beliefs only works with absolute durations for now. I'll add this info to the test.

Low level tests for resampling data should happen in timely-beliefs imo.