C#: Make all classes public

Otherwise Mono will default them to private.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
C#: Throw an exception in get_trigger if the C function returns an error

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
C# examples: Manually destroy the IOBuffer object as soon as possible

If we leave this to the garbage collector, the Context object may be
deleted before the IOBuffer one, which will cause errors at runtime.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
C#: Update IOBuffer to latest libiio API

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