Make a few small updates to support TeXitoi rusty clock (#27)

- add a feature gated alternative full lut for type_a displays
- remove pub from set_lut_helper function
- fix behaviour of set_lut for epd2in9. it always sets the LUT now!
-  better comments

src/epd2in9/mod.rs

@@ -325,6 +325,7 @@ where
         Ok(())
     }
 
+    /// Set your own LUT, this function is also used internally for set_lut
     fn set_lut_helper(&mut self, spi: &mut SPI, buffer: &[u8]) -> Result<(), SPI::Error> {
         assert!(buffer.len() == 30);
         self.interface