Possible performance improvement of TileSet class by serialization.

[HaitaoHuang-USDA]

The object of opened tiles (from open_rasterio function) could be dumped to disk using pickle package.

The running times of loading pickled .pkl files are less than opening raster .bil files. The several tests on Ceres of running times for opening 1000 .bil files and loading 1000 .pkl files:

open 1000 bil files (s)	load 1000 pkl files (s)
500. 2	25.1
193.3	12.8
213.98	4.8
152.5	1.3
207.8	13.9
138.3	1.8
225.6	11.2

The serialization should be done dynamically (if a pkl file for a tile can't be found, then create the pkl file) to avoid missing pkl issue after TileSet dataset updating.

Possible improvements we discussed:

• chunks of xarray
• VRT -- GDAL Virtual Format