convert between SE3 and SO3: use SE3(X) or SO3(X)

can now take angdist between SE3 instances

some changes to Plucker class:

• constructors: PQ is now TwoPoints, Planes is now TwoPlanes
• methods now for point on line closest to given point closest_to_point (was closest) and point on line closest to another line closest_to_line.
• fleshed out angvelxform convert angular velocity to RPY, Euler, exponential coordinate rates
• added angvelxform_dot which maps angular acceleration to RPY, Euler, exponential coordinate acceleration

base/graphics, added ability to plot cuboid, cylinder and homogeneous line. Call signatures are now much more uniform, so quite a few changes but doco is unto date.

base and tests/base is now formatted using black