ENH: parallel tiling

[DPeterK]

#5 and #6 have shown that there is scope to expand the functionality of TDBWriter.tile(). Such extra functionality greatly increases the number of TileDB append operations that need to be carried out.

As append operations can be a little slow, we can improve the speed overall by parallelising the required appends using dask. For example:

writer.tile([nc_files], ['air_temperature', 'rainfall_flux'], ['height', 'time'], parallel=True)