Add the to Convert Varibales to Dimensions

[dzelge]

Is your feature request related to a problem? Please describe.

We need the ability to convert a dataset variable into a dimension as requested by Norman.

Describe the solution you'd like

Something like this:

ds2 = xr.concat([ds[var_name] for var_name in ds.data_vars], "var")
var_coords=xr.DataArray([var_name for var_name in ds.data_vars], dims=["var"])
ds2.assign_coords(var=var_coords)