add at_time overload which accepts lists

[lpdon]

Possible solution for #212 . How the output looks like running the script now:

Trajectory duration: 3.8796 [s]
Position at time 1.0000 [s]: [0.6388750444233515, -1.1631409505208554, -0.11589549559277784]
Positions at times 0.5000 1.0000 1.5000 [s]: [[0.08333333333333333, -0.8314814814814853, 0.20082986124446606], [0.6388750444233515, -1.1631409505208554, -0.11589549559277784], [1.628943788676886, -1.1407581380208773, -0.4326208524300217]]
Position extremas are [[0.000000, 5.000000], [-2.000000, 0.000000], [-3.500000, 0.500000]]