distance marginalisation loookup table made multiple times

Issue

bilby_pipe jobs with the dist marg on have multiple generation jobs making the same dist lookup table (same prior):

grep "Building lookup table for distance marginalisation" out_O2_BCRs/chunk18/pe_outdir/out_background/log_data_generation/*.err

data2146_generation.err:16:53 bilby INFO    : Building lookup table for distance marginalisation.
data2173_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2174_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2176_generation.err:16:54 bilby INFO    : Building lookup table for distance marginalisation.
data2177_generation.err:16:54 bilby INFO    : Building lookup table for distance marginalisation.
data2178_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2179_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2180_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2181_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2190_generation.err:16:53 bilby INFO    : Building lookup table for distance marginalisation.
data2191_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2193_generation.err:16:52 bilby INFO    : Building lookup table for distance marginalisation.
data2195_generation.err:16:53 bilby INFO    : Building lookup table for distance marginalisation.
data2206_generation.err:16:54 bilby INFO    : Building lookup table for distance marginalisation.
data2214_generation.err:16:54 bilby INFO    : Building lookup table for distance marginalisation.

Suggestion

If distance-marginalisation = True, make a parent node (before the generation node) to create a lookup table.

This will ensure that there is only one job that makes a lookup table.