Implement missing commands

Currently following commands are not implemented:

[ ] FORCE_SCAN 0x21: this command is analogous to the SCAN command, but also returns debug information. Not fully documented.
[ ] EXPRESS_SCAN 0x82: will run scanner at the sampling rate as high as it can be. For A2 it should be 4 kHz.
[ ] GET_SAMPLERATE 0x59: returns sampling rates for SCAN and EXPRESS_SCAN commands
[ ] GET_ACC_BOARD_FLAG 0xFF: seems for accessing A2 motor control board. Not documented.

SkoltechRobotics / rplidar