Added API function iio_channel_is_scan_element

This function can be used to detect if a channel is a scan element, that
is to say, if it can be read from / written to.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Local backend: Detect channels that are scan elements

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
XML backend: Detect channels that are scan elements

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