Add forcing height variable to GSWP3 forcing files

[olyson]

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.

[ekluzek]

This work will need to be done in CDEPS. The CTSM part of this will be to update to the CDEPS tag where this is done. I think the way to do this will be to add an additional file for ZBOT, that will be handled as an additional stream file. The file will only have a single time-sample and one latitude and longitude, but it would be a separate stream.

[ekluzek]

I have a branch and PR in CDEPS that fixes the read for ZBOT

ESCOMP/CDEPS#169

[wwieder]

Just clarifying, this will also fix the bug with reading in ZBOT from DATM files for single point runs (adding next so we can discuss).

[ekluzek]

@wwieder the CDEPS PR above fixes the bug for reading ZBOT for single point runs. Using that branch will fix that for you. I've just pinged Jim to approve the PR so we'll have a CDEPS tag for it.