Decide how many decimals to keep for every sensor field

[jdavid]

This is a documentation effort: review and complete the https://github.com/spectraphilic/wasp_sketches/blob/master/docs/sensors.md

Doing this will allows us to:

• Avoid precission errors (see issue #9)
• Reduce the size of frames (see issue #50)
• Save storage space and improve query performance in the server

[ArcticSnow]

When you store the data in the /DATA folder, do you store the value before transformation (x10) or after?

[jdavid]

After, the value stored in the frame is an integer.