ENH: Store in SQL using double precision

Closes #9009 . @jorisvandenbossche -- do you think this should test/use single precision ?  We could test for float32 perhaps? My own feeling is that this is the safer default.
Certainly fine to add this! But can you also add a test for it? (just a test for the API usage, so it won't get removed accidentally, so it can go in `TestSQLApi` (no need to test it for each flavor separately)