Fix is_interpolating()

[708yamaguchi]

Fix is_interpolating() method.

This PR follows roseus impelementation.
https://github.com/jsk-ros-pkg/jsk_pr2eus/blob/16cb8fa58aa1ef7884b01bfdc29c0de1bba76d5a/pr2eus/robot-interface.l#L146

Test code1

ri.angle_vector([0,0,0,0,0,0,0,0,0,0], 3)
time.sleep(2)
print(ri.controller_table['default_controller'][0].is_interpolating())
time.sleep(2)
print(ri.controller_table['default_controller'][0].is_interpolating())
True
False

Test code2

ri.angle_vector([0.5,0,0,0,0,0,0,0,0,0], 3); ri.wait_interpolation()
[False]

Test code 3 (with timeout)

ri.angle_vector([0.0,0,0,0,0,0,0,0,0,0], 3); ri.wait_interpolation(timeout=1)
[True]