Fix receivers not always output in the same order

This happened when receivers were recombined on rank 0 when using MPI.
Need to ensure consistancy in the regression tests when comparing
serial and MPI implementations.

gprMax/fields_outputs.py

@@ -136,6 +136,10 @@ def write_hd5_data(basegrp, grid, is_subgrid=False):
         basegrp["tls/tl" + str(tlindex + 1) + "/Vtotal"] = tl.Vtotal
         basegrp["tls/tl" + str(tlindex + 1) + "/Itotal"] = tl.Itotal
 
+    # Ensure the order of receivers is always consistent (Needed for
+    # consistancy when using MPI with multiple receivers)
+    grid.rxs.sort(key=lambda rx: rx.ID)
+
     # Create group, add positional data and write field component arrays for receivers
     for rxindex, rx in enumerate(grid.rxs):
         grp = basegrp.create_group("rxs/rx" + str(rxindex + 1))