tiles2gpkg_parallel.py: Support multiple raster tables in one Geopackage

[stevendlander]

tiles2gpkg_parallel.py handles tile input in a very simple way; It collects all tiles within the input folder and then places them into one raster tile table. This is fine for continuous data or perhaps a map mosaic, but it is not conformal to the OGC spec for different data sets.

Ideally, tiles2gpkg_parallel.py should have the option to append a tile set to an existing geopackage and thus create a new raster data table within it.