Adding example of image buffer access for CSharp

Contributed by: Dan Mueller <dan.muel@gmail.com>

Change-Id: I7fdcd1f9c46d5ca2e5fa0dc8690351f84f7cf813
Merge topic 'SIMPLEITK-457_AddSharpGetBufferExample' into next

3d2ee061 Adding example of image buffer access for CSharp