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.
The SAUL implementation of the BMx280 currently returns the pressure in hPa.
The sensor, however, actually reads the pressure in Pa, leading to a
significant loss of precision.
Because the operation range of the BME280 and BMP280 is 300 hPa to 1100 hPa,
we can safely increase the precision by a factor of 10 without risk of overflow.
The only downside is that readings are now presented as `e1 Pa` (eg 10206e1 Pa).
