Replace the use_init_interp code with something simpler

[ekluzek]

The use_init_interp code has some utility in not having to spinup SLIM. But, from @marysa it only takes say 6 years to get SLIM spunup, maybe longer for higher latitudes. So spinup for SLIM is really cheap, especially compared to CLM. But, still it's useful to have the utility to interpolate a different case to a new one. As such I think we should leave the code around. But, as some parts of that code are more complex than needed if we keep it around we should replace it with something simpler. The code for SLIM could be very simple, while for CLM some parts of it are complex.