Added implementation of iio_channel_read_raw / iio_channel_write_raw

The prototype changed slightly; now it takes a iio_buffer as source /
destination, and returns a size_t as no error can possibly happen.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added API function iio_channel_convert_inverse

While iio_channel_convert can be used to convert one sample from its
raw format to the host format, iio_channel_convert_inverse allows to
to the exact opposite.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Added API functions iio_channel_read / iio_channel_write

Those functions can be used to demux / mux a stream and convert it
at the same time.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Document a bit more the public API

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>