Make it easier to insert geometries, with documentation and maybe code

In playing with the new SpatiaLite helpers from #385 I noticed that actually populating geometry columns is still a little bit tricky. Here's what I ended up doing:

import httpx, sqlite_utils
db = sqlite_utils.Database("/tmp/spatial.db")
attractions = httpx.get("https://latest.datasette.io/fixtures/roadside_attractions.json?_shape=array").json()
db["attractions"].insert_all(attractions, pk="pk")

# Schema of that table is now:
# CREATE TABLE [attractions] (
#    [pk] INTEGER PRIMARY KEY,
#    [name] TEXT,
#    [address] TEXT,
#    [latitude] FLOAT,
#    [longitude] FLOAT
# )

db.init_spatialite()
db["attractions"].add_geometry_column("point", "POINT")

db.execute("""
    update attractions set point = GeomFromText(
      'POINT(' || longitude || ' ' || latitude || ')', 4326
    )
""")

That last line took some figuring out - especially the need for the SRID of 4326, without which I got this error:

IntegrityError: attractions.point violates Geometry constraint [geom-type or SRID not allowed]

It would be good to both document this in more detail, but ideally also to come up with a more obvious pattern for inserting common types of spatial data.

Also related:

• #398
• #79

Start with the insertion example in this issue and review the existing init_spatialite() and add_geometry_column() APIs, along with related issues #398 and #79. Determine whether the result should be expanded documentation, a simpler insertion pattern, or both; done means the common spatial-data workflow and SRID requirement are clear and supported.