Add Fractional Share Support

In this release, we add support for fractional amounts (qty) and notional values (notional) in POST/v2/orders requests, via submit_order():

• qty is now an optional float kwarg
• notional has been added as an optional float kwarg
• side is now a kwarg (default is "buy")
• type is now a kwarg (default is "market")
• time_in_force is now a kwarg (default is "day")

With these changes, we encourage users to call submit_order() with named arguments instead of positional ones.

For examples, please refer to the docs.