bugfix: schemas/util.sql: bounding_box(): need to explicitly set SRID to make sure the correct value is used

bugfix: schemas/util.sql: bounding_box(): use st_makeenvelope() instead of st_makebox2d() because st_makebox2d() doesn't support geocoordinate wraparound (it is not SRID-aware)

schemas/util.sql: new_world(): removed no longer needed cast to postgis.geometry

schemas/util.sql: bounding_box(): return postgis.geometry instead of postgis.box2d because box2d is not directly used in postgis functions

bugfix: /Makefile: mk_db: plpython3u: use CREATE EXTENSION instead of createlang so that the plpython3u extension is created in addition to the plpython3u language (as it is on Linux)

schemas/util.sql: added new_world()

bugfix: schemas/util.sql: bounding_box(): use util.range instead of numrange to support wraparound ranges for geocoordinates

schemas/util.sql: range(numeric, numeric): use util.range instead of numrange to support wraparound ranges (which use a modulus system such as geocoordinates)

schemas/util.sql: added `range` type (and cast to numrange), which allows wraparound ranges such as for geocoordinates

schemas/util.sql: bounding_box(): documented usage