Add a stream for ZBOT for GSWP3 forcing data

Rather than having ZBOT hardcoded in datm we want to have it read from a file. The file will only have one latitude/longitude and time-sample. Hence, most of this will just be adding this to the streams mechanism for GSWP3.

Note, that the current code doesn't read in ZBOT, like it should, it only overrides it if it isn't read (which it always isn't) and then datm_datamode_clmncep_mod.F90 has this line...

``` fortran
!--- bottom layer height ---
if (.not. associated(strm_z)) Sa_z(n) = 30.0_r8
```

It needs the following to be added before that...

```
call shr_strdata_get_stream_pointer( sdat, 'Sa_z' , strm_z , rc)
if (ChkErr(rc,__LINE__,u_FILE_u)) return
```

And then a fixed stream file added to the stream_definition.xml file for CLMGSWP3v1.Z, to handle Sa_z. It'll be a little simpler than the other blocks since it won't need to handle differing years.