Material model that includes latent heat of melting

Add a material model that includes the effects of latent heat of melting for two materials (peridotite and pyroxenite). Material parameters can depend on the melt fraction. 

Should I try to squash this into one commit?
This doesn't do what you want -- it's just an empty string, not an empty line. You need to write this as "\n\n" to create an empty line. Other than that, thanks for the extensive documentation of the plugin :-)