Adds `epoch` information to the `Interferometer`

When setting data, gives the user the option to provide the epoch (the GPS time at which the data begins). This is then used in the time-shift.

• Rewrites the phase shift operation
• Adds epoch as option when setting the data
• Renames epoch -> center_time in get_interferometer to avoid confusion
• Fix up the documentation of set_data

@paul-lasky @rory-smith @eric.thrane @colm.talbot @moritz.huebner @marcus.lower

See the attached corner plot for GW150914