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`.
This makes life easier when calling e.g. `saul_reg_write()` with data
stored in flash.
As now the signatures for reading and writing differ (in that `const`
qualifier only), `saul_notsup()` is split into `saul_write_notsup()`
and `saul_read_notsup()`. However, one is implemented as a symbol alias
of the other, so that ROM consumption remains unchanged.
