Read current speed

[dogoepp]

Quoting @fedeallocati:

It is possible [to read current speed from the servos], in 3 steps:

• Find for each dynamixel model what is the unit of the current speed field (they may differ for model to model). The issue with this is that not every model is well documented, for example in the Pros I couldn't find it, but maybe with the Dynamixel Software we can get it.
• Add a double field called something like current_speed_unit in the traits of all models, putting the specific value for each one of them.
• Add the functions get_present_speed_m_s and parse_present_speed_m_s both in BaseServo as virtual functions that throw exceptions (like in get_present_position_angle), and in Servo as the implementation and also as static functions available for the models