Add forcing height variable to GSWP3 forcing files

We want to add a forcing height variable to the GSWP3 forcing files and stream text file (and probably eventually to other forcing datasets as well) so that it is read in from the forcing files instead of being hardcoded in the datm code (currently set to 30m). This variable would still be time- and space-invariant. The main reason is to make this variable more visible to users. This would also be consistent with our flux tower forcing files where forcing height is encoded as ZBOT in the datm forcing files.