New domain function for Elizabeth canine mesh

[bergolho]

• Add a new domain function to construct the refined version of the canine mesh from Elizabeth Cherry
• Add a new example that couples a Purkinje network in a fibrotic tissue

[rsachetto]

Lucas, I will not add private functions to the domain library anymore. You can create a file called custom_functions.c and add it to the src/domains_library folder. The build script will identify this file and it will compile it. I will keep only simple domain functions or functions that use public meshes.

[bergolho]

Sachetto, thanks for the update on how I should write the domain functions for private meshes from now on. I already tested the procedure you mentioned above by creating a "custom_functions.c" file inside the src/domains_library folder. But, in order to work properly, I needed to copy and paste the include section from the "domain.c" in the "custom_functions.c". After that, the example file for the canine mesh worked fine.