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.
When generating the doxygen output the temp sens and offset are in the config group.
Since they are internal parameters not to be accessed by users they are placed in the mpu9x650_internal.h file.
