Add option to consider convective heat current in thermal conductivity calculations

[brucefan1983]

This is important for fluids and might also have small effects in some solid structures.

[hityingph]

It might also be important for some glasses or amorphous solids, which have bigger difussion property when compared with the lattice structures.

[brucefan1983]

We are still studying this. I really want to figure out the role of convection from solid to fluid. I also want to extend the spectral decomposition techniques to more general systems. This might lead to a nice paper.

[brucefan1983]

It turned out that the convection part is very small (about 10%) even for liquids. And there is no need to do spectral decomposition for the convection part, as it must be at zero frequency.

[brucefan1983]

will be solved in #449

[brucefan1983]

close as solved.