Make the returned prior explicit

Functionally, this changes nothing, but it makes it clear that the
filled priors are the dictionary `priors`.

tupak/prior.py

@@ -449,6 +449,8 @@ def fill_priors(prior, waveform_generator):
     for key in bad_keys:
         prior.pop(key)
 
+    return prior
+
 
 def write_priors_to_file(priors, outdir):
     """

tupak/sampler.py

@@ -398,7 +398,7 @@ def run_sampler(likelihood, priors=None, label='label', outdir='outdir',
 
     if priors is None:
         priors = dict()
-    fill_priors(priors, likelihood.waveform_generator)
+    priors = fill_priors(priors, likelihood.waveform_generator)
     tupak.prior.write_priors_to_file(priors, outdir)
 
     if implemented_samplers.__contains__(sampler.title()):