Add singular Poisson demo. Resolves: #139 (#263)

Author: jorgensd

_toc.yml

@@ -23,6 +23,7 @@ parts:
         sections:
           - file: chapter2/diffusion_code
           - file: chapter2/heat_code
+      - file: chapter2/singular_poisson
       - file: chapter2/nonlinpoisson
         sections:
           - file: chapter2/nonlinpoisson_code