Previously, `UNIT_G` was used for g-force with the correct symbol `g`,
`UNIT_GR` for gram (as in kilogram) with the incorrect symbol `G` (which
would be correct for Gauss), and `UNIT_GS` for Gauss with symbol `Gs`
(which is an uncommon but correct symbol).
To avoid confusion between G-Force, Gauss, and Gram the units have been
renamed to `UNIT_G_FORCE`, `UNIT_GRAM`, and `UNIT_GAUSS`. In addition,
gram now uses the correct symbol `g` and Gauss uses `G`.
Add a basic SenML module and submodules with support for:
- Encoding SenML values as CBOR using NanoCBOR.
- Converting from Phydat to SenML.
- Reading and encoding SAUL sensors.