Pymycobot

- add methods for raspberry pi version basic control.
- `def gpio_init(self)`
- `def gpio_output(self, pin, v)`

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- sync with Atom3.1

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- improved parameter checking.
- new class `CommandGenerator` that generate binary real command.
- can import needed class from `pymycobot`, like:
python
from pymycobot import Mycobot, Angle, Coord, CommandGenerator


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- fixed `set_pwm_output()`

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2021-05-24

- update demo.

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- set_free_mode -> release_all_servos
- Add new port:
- is_controller_connected
- set_servo_data
- get_servo_data
- set_servo_calibration
- set_basic_output
- Update API document.

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- fix focus_servo error

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