Include the hydrothermal circulation process in visco_plastic section

[sibiaoliu]

I added a new calculation of thermal conductivity caused by lithospheric hydrothermal circulation. This process is commonly used to assess the thermal evolution of the crust and upper mantle at spreading or rifted margins (e.g., mid-ocean ridges). We use the same equation as Gregg et al. (2009). Currently, this function is only implemented in visco_plastic material model.

@naliboff and @anne-glerum Do you think it would be useful to include it in the visco_plastic section? If so, I would submit an pull request for it? Thanks!