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`.
The current phydat_fit implementation the following limitations:
- The API is way more complicated to use than needed
- It doesn't perform any rounding
- It uses `long` in a place where actual width (or better range) of the type
is pretty important.
This commit addresses these limitations and uses lookup-tables to reduce the
number of divisions required.
Before this commit code using it looked like this:
``` C
long values[] = { 100000, 2000000, 30000000 };
phydat_t dat = { .scale = 42, .unit = UNIT_V };
phydat_fit(&dat, values[0], 0, phydat_fit(&dat, values[1], 1, phydat_fit(&dat, values[2], 2, 0)));
```
Now it can be used like this:
``` C
int32_t values[] = { 100000, 2000000, 30000000 };
phydat_t dat = { .unit = UNIT_V, .scale = 42 };
phydat_fit(&dat, values, 3);
```
