Add new function for uploading data per file

[nhoening]

We have post_measurements() which uses the API (POST sensors/data) to send data per JSON, but now we have a new endpoint for sending data per file upload (POST sensors/(id)/data/upload`).

For many devs, that would be a useful way to start.

Also, I would rename the existing one. It can be used to send anything, not just measurements. (and the pendant is already called get_sensor_data()).
Let's call the send_sensor_data() and upload_sensor_data().

[Flix6x]

Counter-offer: a single method named post_sensor_data (post being the counterpart to get) that supports both a file(path) as input or a time series description (values, start and duration, from the top of my head), switching between endpoints accordingly, under the hood (similar to how trigger_and_get_schedule stitches between the single-device and multi-device endpoints corresponding to the provided input).

[nhoening]

Okay, let's use post_sensor_data() (renaming post_measurements() to that). That function decides which of the two API endpoints to talk to, based on parameters.

Challenges:

1. Refactor code so that either start/duration/values, or a file is being used. Complain if neither of them or both is used.
2. Solve problem of sending a file to the endpoint like the browser does.

We might need to adjust tests, and ideally add a new test case for the file upload.