Save the time in a file

When we read the index file, we start first by building the array of time step. It can be long in larger simulation, therefore we should avoid doing it every time.

In this merge request, I am writing this array into a file and restore from it if possible.

logger/examples/reader_example.py

@@ -57,6 +57,7 @@ print("time: %g" % args.time)
 # read the logger
 positions = np.empty((0, 3))
 entropies = np.empty(0)
+gas_type = 0
 for f in args.files:
     if f.endswith(".dump"):
         filename = f[:-5]