`tupak.utils.core.create_time_series` bug

@paul-lasky ,@gregory.ashton ,@colm.talbot The function returns incorrect time series for certain input values. For example:

ts = tupak.utils.create_time_series(sampling_frequency=1000, duration=0.51, starting_time=-0.5)

returns

[-0.5   -0.499 -0.498 ...  0.507  0.508  0.509]

even though it should end with 0.01.

I will create a merge request to fix this today