Don't necessarily overwrite luminosity distance if redshift is supplied

For cosmology studies, it is conceivable that a user will generate their own precomputed dL -> z mapping and save both of these values in the injection parameters, along with source-frame masses. Currently, in this case the conversion function will ignore the dL supplied by the user and overwrite it such that it uses the Planck15 dL given the supplied redshift.

This fix works under the assumption that if the user has supplied both z and dL, they have confirmed that they are related by the intended cosmology a priori.