Make executable script and add header

tools/rebalance-corenrn-data.py

@@ -1,3 +1,13 @@
+#!/bin/env python3
+
+"""
+A post-processing script to redistribute CoreNeuron input files
+more evenly across ranks based on their filesystem size.
+
+Blue Brain Project - EPFL, 2024
+
+"""
+
 import argparse
 import heapq
 import itertools