Add cacheing of marginalized luminosity-distance lookup table

If a user chooses the bilby default prior for luminosity distance, we could have the lookup table saved so that it can be just loaded in. I think it would be useful in some cases where people are interested to play around with their likelihood but don't want to wait for the lookup table to be built each time.

EDIT: (Greg) I changed the title to describe the solution discussed in the comments below