provide informative error message and some relative tolerance to InterferometerList._check_interferometers:

I am getting the following error from bilby:

ValueError: The start_time of all interferometers are not the same

This MR adds the actual values that are miss-matched between the IFOs to this ValueError's message. Eg:

ValueError: The start_time of all interferometers are not the same (H1[start_time]=0, L1[start_time]=1)

This MR will also add some relative tolerance to the values stored in the IFOs (it will accept values, with a large warning, if the values are the same up to 5 dp)