Adding outdir support for ultranest's mpi functionality

Similar to pymultinest, do not use temporary outdir when the sampler is running with mpi

bilby/core/sampler/ultranest.py

@@ -84,6 +84,12 @@ class Ultranest(_TemporaryFileSamplerMixin, NestedSampler):
         callback_interval=10,
         **kwargs,
     ):
+        try:
+            from mpi4py import MPI
+
+            using_mpi = MPI.COMM_WORLD.Get_size() > 1
+        except ImportError:
+            using_mpi = False
         super(Ultranest, self).__init__(
             likelihood=likelihood,
             priors=priors,
@@ -97,6 +103,13 @@ class Ultranest(_TemporaryFileSamplerMixin, NestedSampler):
             **kwargs,
         )
         self._apply_ultranest_boundaries()
+        self.exit_code = exit_code
+        if using_mpi and temporary_directory:
+            logger.info(
+                "Temporary directory incompatible with MPI, "
+                "will run in original directory"
+            )
+        self.use_temporary_directory = temporary_directory and not using_mpi
 
         if self.use_temporary_directory:
             # set callback interval, so copying of results does not thrash the