Fix whitespace in AP_MotorsHeli.*
Rename AP_MOTORS_HELI_SWASH_CCPM to AP_MOTORS_HELI_SWASH_H3

This sets the stage for adding AP_MOTORS_HELI_SWASH_H4, which is also
a CCPM mode.
Rename _servo_4 to _servo_yaw

This sets the stage for a fourth swashplate servo.
Add AP_MOTORS_HELI_SWASH_H4
AP_MotorsHeli: Store swash servos in an array