add smoothing parameter option for xsmooth_sem_pde

[bch0w]

SPECFEM3D now has the option for diffusion based PDE smoothing which is much faster than the previous Gaussian convolution based filtering (SPECFEM/specfem3d#1725). Need to add an option in the parameter file so that the User can use this version of smoothing. Similar to how laplacian smoothing is implemented in Globe.

[bch0w]

closing, redundant #236