Versions of constants

During the bilby review, we noticed differences between the constants defined in LALSuite and those which we where using (which where based on astropy. The thread containing the discussion can be found here: bilby#408 (comment 116119)

The short version of @colm.talbot's digging is that the constants in LAL are based on CODATA2010, but there is n updated CODATA2018 which has slightly different numbers.

In the interim, we decided in bilby to fix on the LAL constants for consistency, but ultimately the constants in LALSuite should be up to date.

cc @vivien @john-veitch @colm.talbot @paul.lasky