Visual-kinematics

- architechture improved: Robot becomes an abstruct class. New class RobotTrajectory is added.
- support forward/inverse kinematics, trajectory visualization as well as workspace analysis for delta robots

- numerical inverse method improved: can choose between ***jac_pinv*** and ***jac_t***

- add support for modified dh parameters
- add interpolation function: robot.interpolate(tra, num_segs, motion="p2p", method="linear")

Auto-installing all the dependencies.

Update example codes and images.

Made it capable of 7 DoF arm as well.