Don't allow REFLINE mask to prevent finding reference lines

The centroidLines.mask defaults to ["BAD", "SAT", "CR", "REFLINE", "NO_DATA"]. This is odd; REFLINE is book-keeping. Maybe this made sense when it was only used for trace removal (but as that comes before line finding I don't quite see how it worked).

This is probably just an oddity, except that the plane isn't cleared when reduceExposure.useCalexp is True. So this ticket has two purposes:

• discuss whether it's a good idea to use REFLINE like this
• clear the mask on read.

Paul will want to see this, but Jira won't let me link him