Updates the method used to create frequency array

Ensures the resulting array is always odd as expected.

@colm.talbot could you check this is correct? Specifically, it fixes the error caused when using signal_ifo *= np.exp(1j*2*np.pi*time_shift*self.source.frequency_array) in the likelihood when the number of time's is even. I wrote the original piece of code and didn't appreciate the must be odd requirement.