Upgrade interaction to support different kernels on different subdomains

[drwells]

From a conversation with @boyceg:

We may want to use a "big" kernel function for the myocardium to improve stability
but a "small" kernel function for the valve leaflets and vessel walls to reduce numerical resistance

This might not be that hard to do - we just need to set up NodalPatchMap objects for each relevant subdomain and make sure we don't double-count any nodes.