Untimed trajectory has exception when calling dq

Summary

When initializing a trajectory from q only, it does not work as expected.

Steps to reproduce

from timor.utilities.trajectory import Trajectory
import numpy as np
arr = np.random.rand(100)
t = Trajectory(q=arr)
t.dq  # Or anything else that might internally need this. For me, it was t.plot()

What is the current bug behavior?

I would expect methods such as t.plot() not to raise an exception after succesfull initialization.

What is the expected correct behavior?

I would probably expect dq and ddq to be None if t is not specified. All other methods such as sum and plot should still work in that case.

Possible fixes

I think a Lazy eval that can deal with t=None would be one possible solution.

/cc @ga59peh