feat: add Euler, Heun and DPM++ (2M) samplers (#50)

* Add Euler sampler

* Add Heun sampler

* Add DPM++ (2M) sampler

* Add modified DPM++ (2M) "v2" sampler.

This was proposed in a issue discussion of the stable diffusion webui,
at https://github.com/AUTOMATIC1111/stable-diffusion-webui/discussions/8457
and apparently works around overstepping of the DPM++ (2M) method with
small step counts.

The parameter is called dpmpp2mv2 here.

* match code style

---------

Co-authored-by: Urs Ganse <urs@nerd2nerd.org>
Co-authored-by: leejet <leejet714@gmail.com>

stable-diffusion.h

@@ -17,7 +17,12 @@ enum RNGType {
 };
 
 enum SampleMethod {
-    EULAR_A,
+    EULER_A,
+    EULER,
+    HEUN,
+    DPMPP2M,
+    DPMPP2Mv2,
+    N_SAMPLE_METHODS
 };
 
 class StableDiffusionGGML;