Allow rescaled priors from gracedb

This MR will automatically do the ROQ scaling for events from gracedb.

• There's a new template for online priors. This takes mass and distance bounds.
• The prior file is written to the output directory and then used from there.
• The distance marginalisation lookup table isn't currently working as expected...
• Low and high-mass events will be scaled.
• I extended the timeout time for the ping of Google. It was failing for my connection.
• I added the default distances to utils.
• Some cleanup of the gracedb module with improved help messages (pointing out the reliance on CIT infrastructure)
• Fix bugs with bilby version 0.5.2