* initial raster support
* _st_intersects_geom -> _st_intersects_geom_nband
* add tests
* update docs
* improve docs
As requested by @marionschleifer
* new type for raster values
Suggested by @lexi-lambda
* replace `SEUnsafe "NULL"` with SENull
These changes also add a new type, PGColumnType, between PGColInfo and
PGScalarType, and they process PGRawColumnType values into PGColumnType
values during schema cache generation.