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.
maybe just add here a check that the Float/Real is in the schema

```
type = 'FLOAT' if self.mode == 'sqlalchemy' else 'REAL'
self.assertTrue(type in create_sql)
```

but maybe make a custom df with only ints, so we are sure the float is there because of the `dtype` argument and not because it was already there (because now, the schema will not be different with or without using the dtype argument)