SCMI Writer raises exception with lettered grids

[djhoese]

Describe the bug
The AWIPS SCMI writer has an option to tile data on a predefined "lettered" grid where each tile is assigned a letter. This makes it easier for forecasting offices to filter downloaded/received data by what tiles they want. I was recently testing this functionality with my geo2grid project and noticed that the lettered tile generation is broken.

To Reproduce

scn.save_datasets(writer='scmi', lettered_grid=True)

Expected behavior
Multiple lettered tiled netcdf files.

Actual results

  File "/Users/davidh/repos/git/satpy/satpy/scene.py", line 1028, in save_datasets
    return writer.save_datasets(datasets, compute=compute, **save_kwargs)
  File "/Users/davidh/repos/git/satpy/satpy/writers/scmi.py", line 892, in save_datasets
    for (trow, tcol, tile_id, tmp_x, tmp_y), tmp_tile in tile_gen(data, fill_value=fill_value):
  File "/Users/davidh/repos/git/satpy/satpy/writers/scmi.py", line 253, in __call__
    for tile_info in tile_infos:
  File "/Users/davidh/repos/git/satpy/satpy/writers/scmi.py", line 382, in _generate_tile_info
    tile_id = self._tile_identifier(gy, gx)
  File "/Users/davidh/repos/git/satpy/satpy/writers/scmi.py", line 364, in _tile_identifier
    alpha = string.ascii_uppercase[alpha_num]
IndexError: string index out of range