[BUG] Non-intuitive behavior if periodic not set as list

[dhruvbalwada]

If the periodic argument to Grid is not passed as a list then the resultant grid object has non-intuitive result.

In example below user may expect both object to have only X dim as periodic. However, in case where list is given X dim is periodic and Z is non-periodic, while when the dimension is specified as string the option True is likely being used.

import xarray as xr
import numpy as np
from xgcm import Grid

ds = xr.Dataset(
    coords={
        "zl": (["zl"], np.arange(1,15)),
        "zi": (["zi"], np.arange(0.5, 15.5)),
        "xl": (["xl"], np.arange(1,15)),
        "xi": (["xi"], np.arange(0.5, 15.5)),
    }
)

grid_list = Grid(ds, 
            coords={"Z": {"center": 'zl', 'outer': 'zi'},
                    "X": {"center": 'xl', 'outer': 'xi'}},
           periodic=['X'])

grid_non_list = Grid(ds, 
            coords={"Z": {"center": 'zl', 'outer': 'zi'},
                    "X": {"center": 'xl', 'outer': 'xi'}},
           periodic='X')

The solution would be to raise an error to the user and force them to use a list, or automatically convert string to a list of a single value.