Add subdatasets function

[acoque]

Add a way to easily access subdatasets (see #463):

julia> dataset = ArchGDAL.read("path/to/S2A_MSIL1C_xxx.SAFE/MTD_MSIL1C.xml")
GDAL Dataset (Driver: SENTINEL2/Sentinel 2)
File(s):
  path/to/S2A_MSIL1C_xxx.SAFE/MTD_MSIL1C.xml

julia> ArchGDAL.subdatasets(dataset)
4-element Vector{String}:
 "SENTINEL2_L1C:path/to" ⋯ n bytes ⋯ "E/MTD_MSIL1C.xml:10m:EPSG_xxx"
 "SENTINEL2_L1C:path/to" ⋯ n bytes ⋯ "E/MTD_MSIL1C.xml:20m:EPSG_xxx"
 "SENTINEL2_L1C:path/to" ⋯ n bytes ⋯ "E/MTD_MSIL1C.xml:60m:EPSG_xxx"
 "SENTINEL2_L1C:path/to" ⋯ n bytes ⋯ "E/MTD_MSIL1C.xml:TCI:EPSG_xxx"

julia> ArchGDAL.subdatasets(dataset)[1]
"SENTINEL2_L1C:path/to/S2A_MSIL1C_xxx.SAFE/MTD_MSIL1C.xml:10m:EPSG_xxx"

julia> subdataset = ArchGDAL.read(ArchGDAL.subdatasets(dataset)[1])
GDAL Dataset (Driver: SENTINEL2/Sentinel 2)
File(s):
  path/to/S2A_MSIL1C_xxx.SAFE/MTD_MSIL1C.xml
  path/to/S2A_MSIL1C_xxx.SAFE/GRANULE/L1C_xxx/MTD_TL.xml
  path/to/S2A_MSIL1C_xxx.SAFE/GRANULE/L1C_xxx/IMG_DATA/xxx_B04.jp2
  path/to/S2A_MSIL1C_xxx.SAFE/GRANULE/L1C_xxx/IMG_DATA/xxx_B03.jp2
  path/to/S2A_MSIL1C_xxx.SAFE/GRANULE/L1C_xxx/IMG_DATA/xxx_B02.jp2
  path/to/S2A_MSIL1C_xxx.SAFE/GRANULE/L1C_xxx/IMG_DATA/xxx_B08.jp2
  ...

Dataset (width x height): 10980 x 10980 (pixels)
Number of raster bands: 4
  [GA_ReadOnly] Band 1 (Red): 10980 x 10980 (UInt16)
  [GA_ReadOnly] Band 2 (Green): 10980 x 10980 (UInt16)
  [GA_ReadOnly] Band 3 (Blue): 10980 x 10980 (UInt16)
  ...

If there is no subdataset, ArchGDAL.subdatasets function returns Vector{String}().