Adding new file with Cahn Hilliard extended to Flory Huggins binary mixture. This was used in email to Chris.

Amended Cahn Hilliard code in a minimal way to account for Flory Huggins binary mixture with the corresponding mobility scaling. Seems to work, but not sure about the volume fraction to concentration conversion.

2D/1D Flory Huggins works in principle. Currently output only via paraview, need to figure out plotting from fenics.