Support more dx and dy resolution options

[zxdawn]

The dx or dy in the doc is 2km and the default value is 10km. I tested 20km and found the resolution is limited to 10 km.

This is caused by: https://github.com/deeplycloudy/glmtools/blob/b70484c79f12a3fdfcc8f5077ad7f37bf4caa594/examples/grid/make_GLM_grids.py#L137-L146

And that is related to the pre-defined goesr_resolutions in lmatools: https://github.com/deeplycloudy/lmatools/blob/731a330494544bf80c7f6752bae67d8f35cb806a/lmatools/grid/fixed.py#L58-L66

So, if I need 20 km, then I have to add the support of 20km in lmatools first:

goesr_resolutions = {
    '0.5km': 14e-6,
    '1.0km': 28e-6,
    '2.0km': 56e-6,
    '4.0km': 112e-6, 
    '8.0km': 224e-6, # not technically in the spec, but SYMMETRY.
    '10.0km': 280e-6, 
    '20.0km': 560e-6, 
    '100.0km': 2800e-6,
}

then, add 20 km to the .py script:

def nearest_resolution(args):
    """ Uses args.dx to find the closest resolution specified by the
    GOES-R PUG. Returns something like "10.0km" that can be used as the
    resolution argument to get_GOESR_grid.
    """
    goes_resln_options = np.asarray([0.5, 1.0, 2.0, 4.0, 8.0, 10.0, 20.0])