including smoothing when building the initial conditions fields

[dksasaki]

I believe it will be useful to smooth the initial condition fields before starting the model. I have had improved results with this choice in MOM6 (for instance) and will be hardcode it in pyroms_tools/initial/ic_file_build.py for now. I'll be using a gaussian filter in order to implement it.

[dksasaki]

I included the gaussian filters here - I only did it for 3D fields for now - perhaps we can do it only for the 3D fields? I will test it before further changing the code.