Add MCMC combine method and improve shuffle behaviour when combining results

Cherry picked results from !945 (merged)

Note: this will change the default behaviour when combining both MCMC and nested samples. Instead of shuffling the results together, they will be unshuffled enabling converge checks of the individual chains.

• Add option to shuffle posteriors when combining (default to false)
• Add mcmc sample combine

test/integration/sample_from_the_prior_test.py

@@ -90,10 +90,11 @@ class Test(unittest.TestCase):
             likelihood=likelihood,
             priors=priors,
             sampler="dynesty",
-            npoints=1000,
-            walks=100,
+            nlive=1000,
+            nact=10,
             outdir=self.outdir,
             label=label,
+            save=False
         )
         pvalues = [
             ks_2samp_wrapper(